Studio: Remove wp-content on import

[kozer]

Fixes https://github.com/Automattic/dotcom-forge/issues/8713

Proposed Changes

This PR removes the wp-content folder on import to avoid merging the current content with the incoming imported content.
NOTE: We keep database, db.php and mu-plugins folder to avoid messing up with sqlite-integration-plugin and the rest of mu-plugins that are needed by Studio

Testing Instructions

1. Create a new site.
2. Ensure that you have 2 backups. 1 should have less plugins installed than the other. ( Same can be done with themes , this is an easy way to check if we merging things or not ).
3. Import each on of the backups sequentially. Ensure that the last's site imported plugins are preserved.
4. Start/Stop the server, and ensure everything works as expected

Pre-merge Checklist

• Have you checked for TypeScript, React or other console errors?

[wojtekn]

Let's delete those instead of trashing them. Running multiple imports produces many plugins/themes/uploads entries in the trash, which can be confusing. The confirmation window already makes it clear that import will replace all files and the database.

Ideally, we back up the whole site directory before import starts and revert to that state if import fails, but it would be an idea for another issue. The current solution tries to implement that, but not in a robust way.