chore: Remove deprecated legacy search backend

[juliushaertl]

Summary

While further checking #44951 I noticed that the old search endpoint in core is marked as deprecated and not used anywhere anymore. Files was also the only app left registering a provider.

Further the app had no route left anyways.

Was deprecated in Nextcloud 20 so good to remove.

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)

[kesselb]

🧹

[kesselb]

Don't forget to update https://docs.nextcloud.com/server/latest/developer_manual/app_publishing_maintenance/app_upgrade_guide/upgrade_to_30.html.