[RUN AS-IF-FOSS] Remove merge_request_reviewers feature flag [RUN ALL RSPEC]

...ntrollers/projects/merge_requests/creations_controller.rb

app/controllers/projects/merge_requests_controller.rb

app/graphql/types/merge_request_type.rb

app/helpers/merge_requests_helper.rb

app/models/merge_request.rb

app/serializers/merge_request_basic_entity.rb

app/serializers/merge_request_sidebar_extras_entity.rb

app/services/merge_requests/base_service.rb

app/views/layouts/header/_default.html.haml

app/views/projects/merge_requests/_merge_request.html.haml

app/views/shared/issuable/_sidebar.html.haml

changelogs/unreleased/remove-merge-request-reviewer-ff.yml

config/feature_flags/development/merge_request_reviewers.yml

doc/api/merge_requests.md

doc/user/project/merge_requests/getting_started.md

...p/assets/javascripts/approvals/components/mr_edit/app.vue

ee/app/views/shared/issuable/_approvals.html.haml

ee/lib/ee/gitlab/quick_actions/merge_request_actions.rb

ee/spec/features/merge_request/user_sets_approvers_spec.rb

ee/spec/frontend/approvals/components/mr_edit/app_spec.js