chore(core): Remove unused imports in plugins and migrations

Cleans up unused imports across `plugins.py` and a migration file.

Closes #20482

netbox/core/plugins.py

@@ -1,6 +1,5 @@
 import datetime
 import importlib
-import importlib.util
 from dataclasses import dataclass, field
 from typing import Optional
 

netbox/dcim/migrations/0188_racktype.py

@@ -3,9 +3,7 @@ import django.db.models.deletion
 import taggit.managers
 from django.db import migrations, models
 
-import utilities.fields
 import utilities.json
-import utilities.ordering
 
 
 class Migration(migrations.Migration):