Drop checks whether a squash is in progress

What does this MR do?

Originally, Gitaly has implemented squashes via a separate worktree of the repository. This has changed though in v14.1, where Gitaly now uses an in-memory implementation of squashes. The result of this is that it is both faster (~4x) and less error-prone.

One side effect though is that it's not possible to tell anymore whether a squash is in progress or not: there is no on-disk state anymore which Gitaly could check. Because of this, Gitaly has deprecated the RPC and will eventually remove it.

Remove this check on GitLab's side to adapt to this change: it doesn't work anymore anyway. If it needs to resurface, the check would instead need to be implemented in GitLab itself, where any ongoing squashes would need to be recorded as part of the database.

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are 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.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)