* lisp/gnus/nnmail.el (nnmail-article-group): Use mm-delete-duplicates
instead of gnus-remove-duplicates.
* lisp/gnus/message.el (message-remove-duplicates): Remove.
(message-idna-to-ascii-rhs-1): Use mm-delete-duplicates instead of
message-remove-duplicates.
* lisp/gnus/mm-util.el (mm-delete-duplicates): Use `delete-dups' if
available, else use implementation from `delete-dups'.