[media] v4l: Remove documentation for nonexistend input field in v4l2_buffer

The input field in struct v4l2_buffer no longer exists but has been replaced
by a reserved field. Remove the field documentation.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>