Remove obsolete comment about lines order in dropdown_user_spec.js

spec/frontend/filtered_search/dropdown_user_spec.js

-import DropdownUtils from '~/filtered_search/dropdown_utils';
-// TODO: Moving this line up throws an error about `FilteredSearchDropdown`
-// being undefined in test. See gitlab-org/gitlab#321476 for more info.
 import DropdownUser from '~/filtered_search/dropdown_user';
+import DropdownUtils from '~/filtered_search/dropdown_utils';
 import FilteredSearchTokenizer from '~/filtered_search/filtered_search_tokenizer';
 import IssuableFilteredTokenKeys from '~/filtered_search/issuable_filtered_search_token_keys';