Remove unnecessary listing in nnselect-categorize

Remove unnecessary listing in nnselect-categorize

* lisp/gnus/nnselect.el (nnselect-categorize): Remove unneeded extra
list around a category value.
(nnselect-retrieve-headers):
(nnselect-request-expire-articles):
(nnselect-request-set-mark):
(nnselect-request-update-info):
(nnselect-request-thread):
(nnselect-push-info):
(gnus-group-make-permanent-search-group): Accommodate change to
nnselect-categorize.
* lisp/gnus/nnir.el (nnir-run-query): Accommodate change to
nnselect-categorize.