build: Remove configure option --enable-gpg-is-gpg2

* configure.ac (--enable-gpg-is-gpg2): Remove option.
(USE_GPG2_HACK): Remove var.
* common/homedir.c (gnupg_module_name): Remove code for gpg2
installation option.
* g10/keygen.c (generate_keypair): Ditto.
* g10/Makefile.am (noinst_PROGRAMS): Ditto.
* doc/gpg.texi: Ditto.
* doc/gpgv.texi: Ditto.
--

This option and all its build stuff does not make anymore sense.  gpg1
is way too old for anyone to use on a regualar base along with a
standard gpg.  It is better to rename that single gpg (1.4) binary to
gpg1 and adjust any scripts.

g10/keygen.c

@@ -5859,11 +5859,7 @@ generate_keypair (ctrl_t ctrl, int full, const char *fname,
 
       tty_printf ( _("Note: Use \"%s %s\""
                      " for a full featured key generation dialog.\n"),
-#if USE_GPG2_HACK
-                   GPG_NAME "2"
-#else
                    GPG_NAME
-#endif
                    , "--full-generate-key" );
 
       err = parse_key_parameter_string (ctrl, NULL, -1, 0,