fix: Delete orphaned plugin management command for django CMS 4

[fsbraun]

Description

This PR changes the plugin.delete() call of the management command into a django CMS version 4 safe way ('plugin.placeholder.delete_plugin(plugin)`): As of django CMS version 4 plugins must be deleted through their placeholder.

Related resources

• #7813
• #...

Checklist

• [ ] I have opened this pull request against develop-4
• [ ] I have added or modified the tests when changing logic
• [ ] I have followed the conventional commits guidelines to add meaningful information into the changelog
• [ ] I have read the contribution guidelines and I have joined #workgroup-pr-review on Slack to find a “pr review buddy” who is going to review my pull request.