[Dominators] Remove the DeferredDominance class

Summary: After converting all existing passes to use the new DomTreeUpdater interface, there isn't any usage of the original DeferredDominance class. Thus, we can safely remove it from the codebase.

Reviewers: kuhar, brzycki, dmgreen, davide, grosser

Reviewed By: kuhar, brzycki

Subscribers: mgorny, llvm-commits

Differential Revision: https://reviews.llvm.org/D49747

llvm-svn: 339502