ASoC: WCD9310: Remove digital mute.

At the end of playback, framework asks CODEC to mute a particular
audio interface through digital mute callback.  In our case, there
is no need to make this function callback available. At the same
time, this cause ADIE loopback not working because earpiece path
is muted. That's why we need to run hostless PCM to have the
framework unmute earpiece path.

CRs-fixed: 309828
Change-Id: I7d5be840644e4892b4f572b3486258b3f7dd3b24
Signed-off-by: Peter Lohmann <plohmann@codeaurora.org>