am able share a record to user using apex how to unshare a record using apex

here is my code:

vf page
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<apex:page standardController="Job__c"   extensions="Jobformation1usercon">
    <apex:form >
                <apex:selectList size="1" value="{!jobst }" multiselect="false">
                    <apex:selectOptions value="{!Job}"> </apex:selectOptions> 
                     <apex:actionSupport event="onchange" reRender="table,opt1" />
                </apex:selectList>
    
                <apex:selectList size="1" value="{!selectedVal}" multiselect="false">
                <apex:actionSupport event="onchange" reRender="table,opt1" />
                    <apex:selectOptions value="{!Value}">
                    </apex:selectOptions>
                </apex:selectList>
               
                <apex:outputText id="opt12" value="{!selectedVal}"></apex:outputText>
               
               <apex:commandLink action="{!manualShareRead}" value="Share" id="opt1">
                </apex:commandLink>
           
     </apex:form>
</apex:page>

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apex page
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public class Jobformation1usercon
{
 
    
         List<user> con1= new List<user>(); 
         public job__c j {get;set;}
         Public List<string> usertselect{get;set;}
         public string jobst { get;set;}
         public string val{ get;set;}
         public user gp {get;set;}
         public boolean nojb{get;set;}
         public String selectedVal{get;set;}
         public List<job__C> joblist = new List<job__C>();
         public Jobformation1usercon(ApexPages.StandardController controller)
        {
        }
  
    public List<SelectOption> getJob()
     {  
         List<SelectOption> option = new List<SelectOption>();
             
        for(job__c j: [Select id,name From  job__c])
         {
         
             option.add(new SelectOption(j.name,j.name));
             system.debug('option value is '+option);
            
         }
         return option;     
    }
     public List<SelectOption> getValue()
     {  
         List<SelectOption> option = new List<SelectOption>();
             
        for(user gp: [Select id,name From  user  ])
         {
         
             option.add(new SelectOption(gp.name,gp.name));
             system.debug('option value is '+option);
            
         }
         return option;     
    }
                
       public PageReference manualShareRead()
           {
             
               joblist = [select id from job__C where name=:jobst];
               con1 = [select id from user where name=:selectedVal];
               system.debug('PGList value is'+con1);
               system.debug('PGList value is'+joblist);
               Job__Share jobShr  = new Job__Share(); 
               jobShr.ParentId = joblist[0].id;
               jobShr.UserOrGroupId = con1[0].id;
               jobShr.AccessLevel = 'Read';
               jobShr.RowCause = Schema.Job__Share.RowCause.Manual;
              
             Database.SaveResult sr = Database.insert(jobShr,false);
              system.debug('sr value is'+sr);
      
             if(sr.isSuccess())
            {
               system.debug('sr.isSuccess() value:'+sr.isSuccess());
                  system.debug('if sr.isSuccess() block');
               PageReference nextpage = new PageReference('/apex/CustomTeam?id='+joblist[0].id);
             
                return nextpage;
               }
           else {
                  system.debug('inner else sr.isSuccess() block');
                     Database.Error err = sr.getErrors()[0];
                     if(err.getStatusCode() == StatusCode.FIELD_FILTER_VALIDATION_EXCEPTION  && 
                     err.getMessage().contains('AccessLevel')){
                      system.debug(' iner if sr.isSuccess() block');
                      PageReference nextpage = new PageReference('/apex/CustomTeam?id='+joblist[0].id);
                return nextpage;
            }
             else
             {
                      system.debug('outer else sr.isSuccess() block');
               PageReference nextpage = new PageReference('/apex/CustomTeam?id='+joblist[0].id);
                    return nextpage;
               }
             }
         
         }
  
  
       
     }


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here job__c is object name is one text field  am shring a reord to one user here  user is one object  .

if  i want to un share want i need to do please help on this

thanks
hareesh