Revert branch 204720-pghintplan-test

It has been decided that we need to investigate other avenues before
settling on pg_hint_plan, so let's clean up for now.

app/finders/issuable_finder.rb

@@ -339,15 +339,6 @@ class IssuableFinder
       cte << items
 
       items = klass.with(cte.to_arel).from(klass.table_name)
-    elsif Feature.enabled?(:pg_hint_plan_for_issuables, params.project)
-      items = items.optimizer_hints(<<~HINTS)
-        BitmapScan(
-          issues idx_issues_on_project_id_and_created_at_and_id_and_state_id
-            idx_issues_on_project_id_and_due_date_and_id_and_state_id
-            idx_issues_on_project_id_and_updated_at_and_id_and_state_id
-            index_issues_on_project_id_and_iid
-        )
-      HINTS
     end
 
     items.full_search(search, matched_columns: params[:in], use_minimum_char_limit: !use_cte_for_search?)

config/feature_flags/development/pg_hint_plan_for_issuables.yml

----
-name: pg_hint_plan_for_issuables
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/46289
-rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/273528
-milestone: '13.6'
-type: development
-group: group::project planning
-default_enabled: false

spec/finders/issues_finder_spec.rb

@@ -472,10 +472,6 @@ RSpec.describe IssuesFinder do
         it 'returns issues with title and description match for search term' do
           expect(issues).to contain_exactly(issue1, issue2)
         end
-
-        it 'uses optimizer hints' do
-          expect(issues.to_sql).to match(/BitmapScan/)
-        end
       end
 
       context 'filtering by issue term in title' do