v4l2-mc: avoid warning about unused variable
Commit 840f5b0572ea ("media: au0828 disable tuner to demod link in
au0828_media_device_register()") removed all uses of the 'dtv_demod',
but left the variable itself around.
Remove it.
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>