drm/i915/gvt: remove obsolete code for old kvmgt opregion

Current GVT contains some obsolete logic originally cooked to
support the old, non-vfio kvmgt, which is actually workarounds.
We don't support that anymore, so it's safe to remove it and
make a better framework.

Signed-off-by: Jike Song <jike.song@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>