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.

Test: Manual
Bug: 117393458
Change-Id: I6f1a5719a1dff946723c35fde7159ef37fb9cdf7