media: v4l2-ioctl: delete unused v4l2_disable_ioctl_locking

The last user of this 'feature' was the gspca driver. Now that
that driver has been converted to vb2 we can delete this code.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>