Remove old compatibility class GitGarbageCollectWorker

What does this MR do?

In !52064 (merged) we namespaced the GitGarbageCollectWorker worker into Projects::GitGarbageCollectWorker. Nevertheless, at that time we couldn't get rid of the original worker because, according to our guidelines, we can only remove workers on major releases (https://docs.gitlab.com/ee/development/sidekiq_style_guide.html#removing-workers), so we had to wait for %14.0.

In this MR we remove the old compatibility worker class.

Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not needed. (Does this MR need a changelog?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.

Related to #299290 (closed)