[media] v4l2-mc: remove the unused sensor var

This fixes this warning:

	v4l2-mc.c: In function 'v4l2_mc_create_media_graph':
	v4l2-mc.c:60:69: warning: variable 'sensor' set but not used [-Wunused-but-set-variable]

We could solve it the other way: don't do the second loop for
webcams. However, that would fail if a chip would have two sensors
plugged. This is not the current case, but it doesn't hurt to be
future-safe here, specially since this code runs only once during
device probe. So, performance is not an issue here.

Reported-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>