config: deprecate/drop crypto.gpg_path under gmime 2.6/3.0

gmime 3.0 no longer offers a means to set the path for gpg.

Users can set $PATH anyway if they want to pick a
differently-installed gpg (e.g. /usr/local/bin/gpg), so this isn't
much of a reduction in functionality.

The one main difference is for people who have tried to use "gpg2" to
make use of gpg 2.1, but that isn't usefully co-installable anyway.