media: v4l: ctrl: Remove old documentation from v4l2_ctrl_grab

v4l2_ctrl_grab() is documented in the header; there's no need to have a
comment explaining what the function does in the .c file.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>