gpu: nvgpu: Remove IOCTL FREE_OBJ_CTX

gpu: nvgpu: Remove IOCTL FREE_OBJ_CTX

We have never used the IOCTL FREE_OBJ_CTX. Using it leads to context
being only partially available, and can lead to use-after-free.

Bug 1834225

Change-Id: I9d2b632ab79760f8186d02e0f35861b3a6aae649
Signed-off-by: Terje Bergstrom <tbergstrom@nvidia.com>
Reviewed-on: http://git-master/r/1250004
Reviewed-on: http://git-master/r/1258422
Reviewed-by: Martin Gao <marting@nvidia.com>
Tested-by: Martin Gao <marting@nvidia.com>
GVS: Gerrit_Virtual_Submit
Reviewed-by: Eric Chuang <echuang@nvidia.com>