Remove the commit count from the commits API

What does this MR do?

This removes the feature flag for api_commits_without_count.

The feature was added in !43159 (merged), to see if it helped improve the SLI for gitaly canary (https://gitlab.com/gitlab-com/gl-infra/scalability/-/issues/470)

As mentioned in https://gitlab.com/gitlab-com/gl-infra/scalability/-/issues/470#note_424178322 this improved overall performace of the P99 requests for this endpoint. But it did not close the gap between cny and main. Still, since it improved performance, I think we should keep the change.

The feature has been on since 2020-10-05 without issues or reports of the header being missing.

Feature flag issue #254994 (closed)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done