checks: Always enable batched computation of commits

What does this MR do?

With 156ce433 (checks: Implement infrastructure for bulk diff checks, 2021-07-29), we have implemented batched loading of all commits for access checks in contrast to loading commits for each change. This both enables us to do a single RPC call to fetch all changes, and furthermore it allows us to use the quarantine directory to enumerate new commits instead of doing a revision walk. Together, these are a big performance improvement based on how many references a repository has: for gitlab-org/gitlab, the time to compute new commits went down from 19 seconds to about 12 milliseconds.

Remove the feature flag of this important performance optimization. The feature flag had been enabled in production on August 16th and can thus be deemed stable.

Closes #336992 (closed)

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 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.)