Revert "audio: Start proxy when external output is enabled"

This reverts commit 61450f47.
 -Playback thread for proxy is started twice.
  Once from write and next from setParameters. So, the
  active use-cases become 3(first being HiFi and next
  HiFi Low latency). So, after the ringtone(or music)
  is completed only only one of the usecase is removed from
  the active list and the playback thread is not stopped as
  the active usecase list is not empty.
 -Reverted the gerrit to not start the playback thread twice

 CRs-Fixed: 573521

Change-Id: I46d601a4b538434d74faecc54fb704eeae8eaedc