[pbqp] Delete some dead code, NFC.

[pbqp] Delete some dead code, NFC.

I took the opportunity to replace some copy|move constructors|assignment
operators with default implementations.

As a follow-up, I plan on threading unique_ptr<T []> through a few areas
per David Blaikie's advice.

Differential Revision: https://reviews.llvm.org/D24424

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@285018 91177308-0d34-0410-b5e6-96231b3b80d8