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.

doc/gpg.texi

@@ -1476,7 +1476,7 @@ Set what trust model GnuPG should follow. The models are:
 
   @item classic
   @opindex trust-mode:classic
-  This is the standard Web of Trust as used in PGP 2.x and earlier.
+  This is the standard Web of Trust as introduced by PGP 2.
 
   @item direct
   @opindex trust-mode:direct
@@ -2342,9 +2342,11 @@ behavior. Note that this is currently the same thing as
 Reset all packet, cipher and digest options to strict RFC-2440
 behavior.
 
+@ifclear gpgtowone
 @item --rfc1991
 @opindex rfc1991
-Try to be more RFC-1991 (PGP 2.x) compliant.
+Try to be more RFC-1991 (PGP 2.x) compliant.  This option is
+deprecated will be removed in GnuPG 2.1.
 
 @item --pgp2
 @opindex pgp2
@@ -2367,6 +2369,12 @@ This option implies
 @end ifclear
 It also disables @option{--textmode} when encrypting.
 
+This option is deprecated will be removed in GnuPG 2.1.  The reason
+for dropping PGP-2 support is that the PGP 2 format is not anymore
+considered safe (for example due to the use of the broken MD5 algorithm).
+Note that the decryption of PGP-2 created messages will continue to work.
+@end ifclear
+
 @item --pgp6
 @opindex pgp6
 Set up all options to be as PGP 6 compliant as possible. This