v4l2-mc: avoid warning about unused variable

Commit 840f5b05

 ("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>