[media] gspca_main: Remove no longer used users variable

Remove the no longer used / useful users variable, and with that gone
there also is no longer a need to take queue_lock in dev_open.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Jean-Francois Moine <moinejf@free.fr>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>