gpgme_op_delete() does not work with gpg 2.x

[Jeroen Ooms]

A user of the R bindings has reported (and I have confirmed) that
deleting secret keys in gpgme does not work with gnupg2. We call (on
an existing key):

  gpgme_op_delete(ctx, key, 1)

This returns success if the key exists, and when we are running with
GnuPG 1.4 the key is deleted. However when running gpg2 it returns
success but the key is still there.

Downstream issue: https://github.com/jeroen/gpg/issues/3