Use gnus-newsgroup-selection instead of nnselect-artlist

Use gnus-newsgroup-selection instead of nnselect-artlist

* lisp/gnus/nnselect.el (nnselect-retrieve-headers):
(nnselect-request-thread):
(nnselect-retrieve-headers):
(nnselect-close-group): Use the summary local variable
gnus-newsgroup-selection rather than the internal nnselect-artlist
variable.
(nnselect-request-group):
(nnselect-request-move-article):
(nnselect-request-expire-articles):
(nnselect-warp-to-article):
(nnselect-request-set-mark):
(nnselect-request-thread): Don't unnecessarily retrieve the
nnselect-artlist value.