[media] V4L: tw9910: remove bogus ENUMINPUT implementation

tw9910 is a TV decoder, it doesn't have a tuner. Besides, the
.enum_input soc-camera operation is optional and normally not needed.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>