gmime-cleanup: remove obsolete gpg_path configuration option and crypto contexts

Note that we do keep ignoring the gpg_path configuration option,
though, to avoid breakage of existing installations.  It is ignored
like any other unknown configuration option, but we at least document
that it is ignored so that people who find it in their legacy configs
can know that it's safe to drop.

signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>