Remove the request_deadline feature flag

What does this MR do?

The feature has been enabled on GitLab.com for over 24h without issue.

It hasn't had the impact we hoped it would, but it does improve the situation. And the calculated deadline can be reused in other parts of the application that should enforce this limit.

See gitlab-com/gl-infra/scalability#71 (closed) for results

The feature itself was added in !21492 (merged)

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