[media] cx231xx: drop unconditional port3 switching

All switching should be done by i2c mux adapters.
Drop explicit dont_use_port_3 flag.
Drop info message about switch.

Only the removed code in start_streaming is questionable:
It did switch the port_3 flag without accessing i2c in between.

Signed-off-by: Matthias Schwarzott <zzam@gentoo.org>
Reviewed-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>