gmime-cleanup: pass NULL arguments explicitly where GMime 3.0 expects it

Several GMime 2.6 functions sprouted a change in the argument order in
GMime 3.0.  We had a compatibility layer here to be able to handle
compiling against both GMime 2.6 and 3.0.  Now that we're using 3.0
only, rip out the compatibility layer for those functions with changed
argument lists, and explicitly use the 3.0 argument lists.

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