Cleanup `disable_git_http_fetch_writes` feature flag
<!--IssueSummary start--> <details> <summary> Everyone can contribute. [Help move this issue forward](https://handbook.gitlab.com/handbook/marketing/developer-relations/contributor-success/community-contributors-workflows/#contributor-links) while earning points, leveling up and collecting rewards. </summary> - [Close this issue](https://contributors.gitlab.com/manage-issue?action=close&projectId=278964&issueIid=506003) </details> <!--IssueSummary end--> ### Problem Feature flag `disable_git_http_fetch_writes` was created [to disable collection of daily statistics](https://gitlab.com/gitlab-org/gitlab/-/issues/324763#note_530238948), because it was affecting the performance: https://gitlab.com/gitlab-org/gitlab/-/issues/324763+. But later this problem was addressed in https://gitlab.com/gitlab-org/gitlab/-/issues/426270+. The feature flag was disabled on GitLab.com: https://gitlab.com/gitlab-com/gl-infra/scalability/-/issues/883#note_2026515911 There is also a related issue for GitLab Dedicated: https://gitlab.com/gitlab-org/gitlab/-/issues/473689+ ### Proposal Remove this feature flag if it's a safe
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