gmime-cleanup: drop unused gmime #defines and simplify g_mime_init ()

gmime-cleanup: drop unused gmime #defines and simplify g_mime_init ()

Several of these #defines were not actually used in the notmuch
codebase any longer.  And as of GMime 3.0, g_mime_init takes no
arguments, so we can also drop the bogus RFC2047 argument that we were
passing and then #defining away.

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