[MINOR BC BREAK] Syncing: PivotWithRelation -> PivotWithCentralResource

The old names of the class and method were misleading. We don't
actually need any relation. And we don't even need a model instance
as we were returning previously -- the only use of that method was
in TriggerSyncingEvents which would immediately use ::class on the
returned value. Therefore, all we are asking for in this interface
is just the central resource class.

src/ResourceSyncing/Listeners/DeleteAllTenantMappings.php

@@ -18,8 +18,6 @@ use Stancl\Tenancy\Listeners\QueueableListener;
  */
 class DeleteAllTenantMappings extends QueueableListener
 {
-    public static bool $shouldQueue = false;
-
     /**
      * Pivot tables to clean up after a tenant is deleted, in the
      * ['table_name' => 'tenant_key_column'] format.