When deleting a previously deleted model, simply returns

Closes #13588

Basically, 'create_migration_file' method on railties AR model_generator.rb was using this path: "../../migration/templates/create_table_migration.rb" (the template file for table creation migration) as argument for migration_template method at migration.rb. Initially I was going just to fix just this, but I thought it might be interesting to show a message to the user in case the file was already deleted.

Feedback would be much appreciated <3