gpg: Remove options --pgp2 and --rfc1991.
* g10/gpg.c (oRFC1991, oPGP2): Remove (opts): Remove --pgp2 and --rfc1991. * g10/options.h (CO_PGP2, CO_RFC1991): Remove. Remove all users. (RFC2440, PGP2): Remove. Remove all code only enabled by these conditions. * tests/openpgp/clearsig.test: Remove --rfc1991 test.
The use of PGP 2.c is considered insecure for quite some time
now (e.g. due to the use of MD5). Thus we remove all support for
_creating_ PGP 2 compatible messages.