[media] media: v4l2-core: remove the use of V4L2_FL_USE_FH_PRIO flag

Since all the drivers that use `struct v4l2_fh' use the core priority
checking instead of doing it themselves, this flag can be removed.

This patch removes the usage of the flag from v4l2-core.

Signed-off-by: Ramakrishnan Muthukrishnan <ramakrmu@cisco.com>
Reviewed-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>