[GCU] Fix missing backend in dry run (sonic-net#2347)

What I did
Fix sonic-net#11912

How I did it
Add the missing 'remove_backend_tables_from_config' in dry run.

How to verify it
Run unit test and manual test locally.

Author: wen587

generic_config_updater/change_applier.py

@@ -65,6 +65,10 @@ def apply(self, change):
         self.config_wrapper.apply_change_to_config_db(change)
 
 
+    def remove_backend_tables_from_config(self, data):
+        return data
+
+
 class ChangeApplier:
 
     updater_conf = None

tests/generic_config_updater/change_applier_test.py

@@ -281,6 +281,7 @@ def test_apply__calls_apply_change_to_config_db(self):
 
         # Act
         applier.apply(change)
+        applier.remove_backend_tables_from_config(change)
 
         # Assert
         applier.config_wrapper.apply_change_to_config_db.assert_has_calls([call(change)])