Exclude the total headers from the commits API

What does this MR do?

This removes the CountCommits gitaly call that was part of the commits API. A consequence of this is that we had to remove the X-Total and X-Total-Pages pagination headers. We already do this for some ActiveRecord::Relation-pagers.

For now, this change is behind the api_commits_without_count feature flag.

This could help with https://gitlab.com/gitlab-com/gl-infra/scalability/-/issues/470

Feature flag roll-out: #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