Remove copy constructing add item methods from PartitionVector (#759726)

Remove PartitionVector push_back() and insert() methods which copy
construct Partitions objects into the vector.  All the code has already
been changed to dynamically allocate Partition objects and use the
adoption variants of these methods named, push_back_adopt() and
insert_adopt().  Remove the no longer used methods.

Bug 759726 - Implement Partition object polymorphism