gmime-extra: drop compat layer for g_mime_multipart_encrypted_decrypt

gmime-extra: drop compat layer for g_mime_multipart_encrypted_decrypt

In practice, we're going to see this function invoked differently
depending on which gmime we build against.  The compatibility layer
forces our code into the lowest-common-denominator -- unable to make
use of new features even when built against a newer version.

Dropping the compatibility layer paves the way for clearer use of
features from GMime 3.0 in future commits.