i9300: audio: don't disable back mic upon voice call end

Back or Secondary Microphone (Sub Mic) is turned on and off
using noise_suppression parameter from adev_set_parameters
and hence it is not required to disable the back mic explicitly
in voicecall_default_disable

Change-Id: I0959bfb0d49c0ba75795f5374e6f5ba5eac59cf1