fix: Remove avatar and credentials only after user removal

[SystemKeeper]

Summary

When deleting a user, we already call $avatarManager->deleteUserAvatar($this->uid);, which removes the avatars after the user was deleted. Additionally we had a before listener, which removed the avatars before a user was actually deleted. This (alongside the removal of credentials) is tricky when the userbackend denies the user deletion (e.g. LDAP with an active user).

How to test:

• Have a LDAP user with a custom avatar
• Run occ user:delete <ldap-user>
• See The specified user could not be deleted. Please check the logs.
• Check the avatar of the user -> it is gone.

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)
• Labels added where applicable (ex: bug/enhancement, 3. to review, feature component)
• Milestone added for target branch/version (ex: 32.x for stable32)

AI (if applicable)

• The content of this PR was partly or fully generated using AI