Remove check for missing parent calls

Telecom now guarantees that when sending a conference call to an in-call
service, all children will be sent before any parents. Thus, it is
acceptable for children to temporarily have invalid references to
parent calls, but not vice versa.

Bug: 26818778
Change-Id: Id34e4c3a773e6f7c75be8c85ec2642cf05dabe0a