Use default diff executor

app/src/main/java/eu/faircode/email/AdapterMessage.java

@@ -121,8 +121,6 @@ import java.util.Arrays;
 import java.util.Collections;
 import java.util.Date;
 import java.util.List;
-import java.util.concurrent.ExecutorService;
-import java.util.concurrent.Executors;
 
 import javax.mail.Address;
 import javax.mail.internet.InternetAddress;
@@ -184,8 +182,6 @@ public class AdapterMessage extends RecyclerView.Adapter<AdapterMessage.ViewHold
     private DateFormat TF;
     private DateFormat DTF = SimpleDateFormat.getDateTimeInstance(SimpleDateFormat.LONG, SimpleDateFormat.LONG);
 
-    private static ExecutorService executor = Executors.newCachedThreadPool(Helper.backgroundThreadFactory);
-
     private static final List<String> PARANOID_QUERY = Collections.unmodifiableList(Arrays.asList(
             "utm_source",
             "utm_medium",
@@ -3298,7 +3294,6 @@ public class AdapterMessage extends RecyclerView.Adapter<AdapterMessage.ViewHold
         this.debug = prefs.getBoolean("debug", false);
 
         AsyncDifferConfig<TupleMessageEx> config = new AsyncDifferConfig.Builder<>(DIFF_CALLBACK)
-                .setBackgroundThreadExecutor(executor)
                 .build();
         this.differ = new AsyncPagedListDiffer<>(new AdapterListUpdateCallback(this), config);
         this.differ.addPagedListListener(new AsyncPagedListDiffer.PagedListListener<TupleMessageEx>() {