mmt/nvidia: remove crap related to object handles

It's demmt job now to decode ioctls and match them to object handles.
And this code is:
a) wrong - uses subdev+handle combo, instead of cid+handle
   (subdev+handle is not unique)
b) error-prone - see commit "mmt/nvidia: temporary fix for 2nd mmap
   of the same offset"
mmt/nvidia: remove nv-specific post mmap hook

It's not needed anymore.