Removes custom_emoji feature flag

app/assets/javascripts/emoji/index.js

app/controllers/groups/custom_emoji_controller.rb

app/finders/award_emojis_finder.rb

app/finders/groups/custom_emoji_finder.rb

app/graphql/mutations/custom_emoji/create.rb

app/graphql/mutations/custom_emoji/destroy.rb

app/helpers/groups_helper.rb

app/models/custom_emoji.rb

config/feature_flags/development/custom_emoji.yml

doc/api/graphql/custom_emoji.md

doc/user/emoji_reactions.md

lib/gitlab/gon_helper.rb

spec/finders/award_emojis_finder_spec.rb

spec/frontend/emoji/index_spec.js

spec/helpers/groups_helper_spec.rb

spec/models/award_emoji_spec.rb

spec/models/custom_emoji_spec.rb

spec/requests/api/graphql/custom_emoji_query_spec.rb

...equests/api/graphql/mutations/custom_emoji/create_spec.rb

...quests/api/graphql/mutations/custom_emoji/destroy_spec.rb