ASoC: Intel: byt-rt5640: Remove IN2N pin from DAPM route table

I tested couple byt-rt5640 based platforms and they have single-ended
headset microphone connection to IN2P only. I guess IN2N was either defined
by accident or some early platform had floating ground for headset. It's
better to remove IN2N and add a custom route for such a platform if needed.

Signed-off-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>