[mms] Remove Horde_Crypt_Pgp#generateRevocation() (Bug #7375).

This won't work unless using a version of GnuPG that hasn't been
released yet, due to the fact that this command can't be run in batch
mode and disabling pinentry is not possible with any released version.

Not BC breaking since this method has never worked.
[mms] Report XML parsing errors (Request #12164; math.parent@gmail.com).