doc: Final update from master (gnupg 2.1)

* doc/Makefile.am (sources_from_trunk): Remove.
(update-source): Make it a dummy.
* doc/gpg.texi: Update.
* doc/yat2m.c: Update.

Maintaining 3 versions in of the gpg manual in one file is getting
more complicated with 2.1. Thus we stop this now and keep the manual
for 1.4 separate.