audio policy: configurable engine: remove unecessary function

The policy plugin does not connect with any HW and does not need to
perform a backward synchronisation at startup. This patch removes
uncessary function linked with this functionality.

Change-Id: Ie0d729f0eb4ea10d9a8450902dabba1fbbf199ef
Signed-off-by: François Gaffie <francois.gaffie@intel.com>