Eliminate `remove-if-not' that is a cl function.

Eliminate `remove-if-not' that is a cl function.

gnus-util.el (gnus-remove-if): Allow hash table.
gnus-util.el (gnus-remove-if-not): New function.
gnus-art.el (gnus-mime-view-part-as-type): Replace remove-if-not with gnus-remove-if-not.
gnus-score.el (gnus-summary-score-effect): Replace remove-if-not with gnus-remove-if-not.
gnus-sum.el (gnus-read-move-group-name): Replace remove-if-not with gnus-remove-if-not.
gnus-group.el (gnus-group-completing-read): Regard collection as a hash table if it is not a list.