doc: Remove outdated Russian man page.

* configure.ac (DOCBOOK_TO_MAN): Remove.
* doc/gpg.ru.sgml: Remove.
* doc/Makefile.am: Remove all gpg.ru related code.

The man page is outdated and we do not use docbook for a long time
now. If someone wants to revive such a man page, it would be best to
translate the respective parts of the GnuPG manual in git master.

• GnuPG-bug-id: T1652