drivers/iio/light/Kconfig: SENSORS_BH1780 cleanup

Commit 7ef9153d ("misc: delete bh1780 driver") has removed the
Kconfig option SENSORS_BH1780.  Remove the last reference on this
option.

Signed-off-by: Valentin Rothberg <valentinrothberg@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>