fix: Remove useless legacy autoloader

[come-nc]

• Related to Reorder server and apps class loaders cleverly #38835

Summary

Given that:

1. The legacy autoloader is not needed for tests anymore as shown in parent PR chore: Remove useless legacy autoloader for tests #52945
2. The legacy autoloader is not used for OCA classes as apps folders are never added as a valid root
3. The legacy autoloader is not used for legacy classes as since Move legacy classes from PSR0 to PSR4 #20265 they do not match the expected naming pattern

I thereby declare it safe to delete the legacy autoloader and rely on sane modern autoloaders like the composer one instead.
Date of death 2025-05-19, after a long period with no brain activity.

I’m a bit sad to say this is not a noticable perf bump according to my testing, but it cannot hurt and opens the door to autoloader optimizing in the future.

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)

[juliusknorr]

🚀