Fix to handle duplicate call id for conference call

When one of the participant drops remotely soon after
initiating a conference, lower layers will re-use the callId
of terminated participant to the newly formed conference call.
As lower layers ensures that callIds assigned to the calls are
unique, remove the callId checks and invoke <callSessionMergeComplete>
callback with the newly formed conference call.

Change-Id: Iea12d22070b3b34266ba6d41fd85898f60636d2a
CRs-Fixed: 2537853