Remove `vue_merge_request_list` feature flag

app/assets/javascripts/issuable/index.js

app/assets/javascripts/merge_requests/list/index.js

app/assets/javascripts/pages/groups/merge_requests/index.js

.../javascripts/pages/projects/merge_requests/index/index.js

app/controllers/groups_controller.rb

app/controllers/projects/merge_requests_controller.rb

app/views/groups/merge_requests.html.haml

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

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

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

config/feature_flags/beta/vue_merge_request_list.yml

...ews/projects/merge_requests/_bulk_update_button.html.haml

spec/controllers/groups_controller_spec.rb

spec/controllers/projects/merge_requests_controller_spec.rb

...roups/user_sees_users_dropdowns_in_issuables_list_spec.rb

spec/frontend/filtered_search/dropdown_user_spec.js

spec/frontend/filtered_search/dropdown_utils_spec.js

spec/frontend/fixtures/merge_requests.rb

spec/frontend/fixtures/static/merge_request_list.html