It's been dead code since forever since mode groups haven't ever been
implemented. On top of that it's also been non-functional since we
only ever filtered the getresources ioctl and not any of the others
nor the mode object lookup code.

Given overwhelming evidence it looks like this isn't a feature we
need, hence remove it.
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>