Merge changes made in Gnus trunk.

Merge changes made in Gnus trunk.

gnus-bookmark.el: Remove useless gnus-bookmark-after-jump-hook.
gnus-group.el: Remove useless gnus-group-icon-cache.
gnus-group.el: Remove useless gnus-ephemeral-group-server.
gnus-picon.el: Remove useless gnus-picon-setup-p.
gnus-sum.el: Remove useless gnus-newsgroup-none-id.
gnus-uu.el: Remove gnus-uu-shar-file-name.
gnus.el: Remove useless gnus-use-generic-from.
gnus.el: Remove obsolete variable gnus-topic-indentation.
mml1991.el: Remove useless mml1991-verbose.
mml2015.el: Remove mc-default-scheme and mc-schemes useless variables.
nnheader.el: Remove useless variables news-reply-yank-message-id and news-reply-yank-from.
rfc1843.el: Remove useless rfc1843-old-gnus-decode-header-function variable.