Clean up common_utils.js (!7318)

app/assets/javascripts/application.js

app/assets/javascripts/gfm_auto_complete.js.es6

app/assets/javascripts/lib/utils/common_utils.js

app/assets/javascripts/merge_request_tabs.js

changelogs/unreleased/cleanup-common_utils-js.yml

spec/javascripts/application_spec.js

spec/javascripts/fixtures/application.html.haml