Remove issue_date_filter feature flag and enable date filtering for all users

Gatla Vishweshwar Reddyrequested to merge
gitlab-community/gitlab-org/gitlab:remove-issue-date-filter-feature-flag into master

What does this MR do and why?

Removes the `issue_date_filter` feature flag and enables date filtering for all users by default.

The feature was introduced in !120160 (merged) and has been behind a feature flag since GitLab 16.2. This MR completes the rollout by:

- Deleting the feature flag definition file

- Hardcoding `true` where the flag was checked in the helper and GraphQL type

  • Removing all feature flag related tests

Closes #17758

References

How to set up and validate locally

  1. Visit any issues list page e.g. http://127.0.0.1:3000/dashboard/issues
  2. Confirm the date filter tokens (Created after, Created before, Closed after, Closed before) are visible in the filtered search bar without enabling any feature flag

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