Globally sanitize single-file package long descriptions (Bug#37548)

Globally sanitize single-file package long descriptions (Bug#37548)

Consistent with multi-file package descriptions which don’t have
commentary sections nor double semicolon prefixes.
* lisp/emacs-lisp/lisp-mnt.el (lm-commentary): Remove commentary
header, double semicolon prefixes of each line, trailing new-lines and
trailing white-space from commentary.
* lisp/emacs-lisp/package.el (package--get-description)
(describe-package-1):
* lisp/finder.el (finder-commentary):
* lisp/info.el (Info-finder-find-node): Remove ad-hoc sanitation.