Drop some epg/epa backwards compat code for ancient Emacs versions

Drop some epg/epa backwards compat code for ancient Emacs versions

* lisp/epa.el (epa--select-safe-coding-system): Remove,
`select-safe-coding-system' has been present since Emacs 20.3.
(epa--encode-coding-string, epa--decode-coding-string):
* lisp/epg.el (epg--decode-coding-string, epg--encode-coding-string)
(epg--clear-string): Remove, the corresponding Emacs functions are so
old they don't show up in NEWS.  Furthermore, the fallback for
the *-coding-string functions was `identity' which wouldn't have
worked anyway, since it only takes a single argument.
(epg--make-temp-file): Remove, `make-temp-file' has been present since
Emacs 21.1.
* lisp/epg-config.el (epg-gpg-program, epg-gpgsm-program)
(epg-gpgconf-program, epg-gpg-home-directory)
(epg-passphrase-coding-system, epg-debug): Remove unneeded :group
parameter.