[Dominators] Remove misleading double-deletion test

Summary:
It's generally not safe to perform multiple DomTree updates without using the incremental API.

Although it is supposed to work in this particular case, the testcase is misleading/confusing, and it's better to remove it.

Reviewers: dberlin, brzycki, davide, grosser

Reviewed By: davide

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D42333

llvm-svn: 323058