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, because it was affecting the performance: Improve the performance of the Projects::FetchS... (#324763 - closed).

But later this problem was addressed in Improve increments of `project_daily_statistics` (#426270 - closed).

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: [SPIKE] Evaluate deprecation of project statist... (#473689)

Proposal

Remove this feature flag if it's a safe