gmime-cleanup: drop all arguments unused in GMime 3

gmime-cleanup: drop all arguments unused in GMime 3

This means dropping GMimeCryptoContext and notmuch_config arguments.

All the argument changes are to internal functions, so this is not an
API or ABI break.

We also get to drop the #define for g_mime_3_unused.

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