Omit edition numbers and dates in manuals

Omit edition numbers and dates in manuals

These seem to cause more confusion than they cure; e.g.,
https://shop.fsf.org/books/gnu-emacs-manual-18th-edition-v-261
currently advertises "18th edition" even as it points to
https://www.gnu.org/software/emacs/manual/pdf/emacs.pdf
which says "Seventeenth Edition".  It is simpler and less
confusing to stick to one version number like "26.3".  If we
need a separate edition number for each printed manual for
some reason (marketing?) we can put “@ifset printed-edition”
around anything specific to the printed editions.
(as opposed to online PDF copies).
* doc/emacs/emacs.texi (EDITION):
* doc/lispintro/emacs-lisp-intro.texi (edition-number, update-date):
* doc/lispref/elisp.texi (VERSION, DATE):
Remove, and remove uses.