Remove old Gnus compat code from MH-E

Remove old Gnus compat code from MH-E

* lisp/mh-e/mh-gnus.el (mh-mm-merge-handles)
(mh-mm-set-handle-multipart-parameter, mh-mm-inline-text-vcard)
(mh-mm-possibly-verify-or-decrypt)
(mh-mm-handle-multipart-ctl-parameter, mh-mm-readable-p)
(mh-mm-long-lines-p, mh-mm-keep-viewer-alive-p)
(mh-mm-destroy-parts, mh-mm-uu-dissect-text-parts)
(mh-mml-minibuffer-read-disposition): Make into obsolete aliases
for functions without the 'mh-' prefix.  Update callers.
(mh-gnus-local-map-property): Remove Emacs 20 compat code.