Remove cached_loading_hints feature flag

Enables caching of the loading_hints partial universally.

Changelog: changed

app/views/layouts/_loading_hints.html.haml

-= cache_if(Feature.enabled?(:cached_loading_hints, current_user), [ActionController::Base.asset_host, user_application_theme, user_color_scheme], expires_in: 1.minute) do
+= cache([ActionController::Base.asset_host, user_application_theme, user_color_scheme], expires_in: 1.minute) do
   - css_crossorigin = ActionController::Base.asset_host ? 'anonymous' : nil
 
   - if ActionController::Base.asset_host

config/feature_flags/development/cached_loading_hints.yml

----
-name: cached_loading_hints
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/61609
-rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/330802
-milestone: '13.12'
-type: development
-group: group::source code
-default_enabled: false