Fix byte compiler warnings.

Fix byte compiler warnings.
(nnir-retrieve-headers): Don't set let-bound vars to nil.  Replace
mapcar called for effect with mapc.
(nnir-compose-result): Bind `group'.  Don't set let-bound vars to nil.
(nnir-run-imap, nnir-artlist-groups): Replace mapcar called for effect
with mapc.
(nnir-run-swish++, nnir-run-swish-e, nnir-run-namazu): Don't set
let-bound vars to nil.