Clean up :apollo_boards feature flag

app/assets/javascripts/boards/index.js

app/assets/javascripts/graphql_shared/issuable_client.js

...p/assets/javascripts/boards/components/board_new_epic.vue

...avascripts/boards/components/board_settings_wip_limit.vue

ee/app/assets/javascripts/boards/components/epic_lane.vue

.../assets/javascripts/boards/components/epics_swimlanes.vue

...assets/javascripts/boards/components/issues_lane_list.vue

.../javascripts/boards/components/toggle_epics_swimlanes.vue

ee/app/assets/javascripts/epic_boards/index.js

ee/spec/frontend/boards/board_card_inner_spec.js

ee/spec/frontend/boards/board_list_helper.js

...c/frontend/boards/components/board_add_new_column_spec.js

ee/spec/frontend/boards/components/board_app_spec.js

ee/spec/frontend/boards/components/board_content_spec.js

.../frontend/boards/components/board_filtered_search_spec.js

ee/spec/frontend/boards/components/board_list_spec.js

ee/spec/frontend/boards/components/board_new_epic_spec.js

...frontend/boards/components/board_settings_sidebar_spec.js

...ontend/boards/components/board_settings_wip_limit_spec.js

ee/spec/frontend/boards/components/board_top_bar_spec.js