nnimap.el: Clean up "unused var" warnings

nnimap.el: Clean up "unused var" warnings

(auth-source-creation-prompts): Declare.
(nnimap-retrieve-headers, nnimap-status-message)
(nnimap-request-create-group, nnimap-request-delete-group)
(nnimap-close-group, nnimap-request-move-article)
(nnimap-request-accept-article, nnimap-request-newgroups)
(nnimap-request-post, nnimap-dummy-active-number)
(nnimap-save-mail-spec, nnimap-get-groups): Add _ to unused vars.
(nnimap-parse-flags): Remove unused var `p'.
(nnimap-retrieve-group-data-early): Remove unused var `groups'.
(nnimap-flags-to-marks): Remove unused var `totalp'.