checks: Always enable batched computation of commits

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.

Changelog: performance