Obsolete gnus-remove-if and gnus-remove-if-not

Obsolete gnus-remove-if and gnus-remove-if-not

Use seq-remove and seq-filter, respectively, instead.

* lisp/gnus/gnus-util.el: Remove functions and replace with
  define-obsolete-function-alias calls.
* lisp/gnus/gnus.el: Require seq here.
* lisp/gnus/gnus-art.el (gnus-mime-view-part-as-type):
* lisp/gnus/gnus-icalendar.el (gnus-icalendar-event--get-attendee-names):
* lisp/gnus/gnus-score.el (gnus-summary-score-effect):
* lisp/gnus/gnus-sum.el (gnus-read-move-group-name):
* lisp/gnus/gnus-topic.el (gnus-group-prepare-topics):
* lisp/gnus/gnus-win.el (gnus-get-buffer-window):
* lisp/gnus/nnmail.el (nnmail-purge-split-history):
* lisp/gnus/nnmaildir.el (nnmaildir-request-scan):
* lisp/gnus/nnrss.el (nnrss-make-hash-index): Replace calls in all
  these locations.