Cleanup `disable_git_http_fetch_writes` feature flag
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### 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
issue