[GCU] Fix missing backend in dry run

[wen587]

What I did

Fix sonic-net/sonic-buildimage#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.

Previous:

admin@vlab-01:~$ sudo config apply-patch t2.json -d
** DRY RUN EXECUTION **
Patch Applier: Patch application starting.
Patch Applier: Patch: [{"op": "remove", "path": "/VLAN/Vlan250"}]
Patch Applier: Getting current config db.
Patch Applier: Simulating the target full config after applying the patch.
Patch Applier: Validating target config does not have empty tables, since they do not show up in ConfigDb.
Patch Applier: Sorting patch updates.
Patch Applier: The patch was sorted into 1 change:
Patch Applier:   * [{"op": "remove", "path": "/VLAN/Vlan250"}]
Patch Applier: Applying 1 change in order:
Patch Applier:   * [{"op": "remove", "path": "/VLAN/Vlan250"}]
** DryRun: Would apply [{"op": "remove", "path": "/VLAN/Vlan250"}]
Patch Applier: Verifying patch updates are reflected on ConfigDB.
Failed to apply patch
Usage: config apply-patch [OPTIONS] PATCH_FILE_PATH
Try "config apply-patch -h" for help.

Error: 'DryRunChangeApplier' object has no attribute 'get_config_without_backend_tables'

After fix:

admin@vlab-01:~$ sudo config apply-patch t2.json -d
** DRY RUN EXECUTION **
Patch Applier: Patch application starting.
Patch Applier: Patch: [{"op": "remove", "path": "/VLAN/Vlan250"}]
Patch Applier: Getting current config db.
Patch Applier: Simulating the target full config after applying the patch.
Patch Applier: Validating target config does not have empty tables, since they do not show up in ConfigDb.
Patch Applier: Sorting patch updates.
Patch Applier: The patch was sorted into 1 change:
Patch Applier:   * [{"op": "remove", "path": "/VLAN/Vlan250"}]
Patch Applier: Applying 1 change in order:
Patch Applier:   * [{"op": "remove", "path": "/VLAN/Vlan250"}]
** DryRun: Would apply [{"op": "remove", "path": "/VLAN/Vlan250"}]
Patch Applier: Verifying patch updates are reflected on ConfigDB.
Patch Applier: Patch application completed.
Patch applied successfully.

Previous command output (if the output of a command-line utility has changed)

New command output (if the output of a command-line utility has changed)