[RemoveDIs] Remove tests for redundant DPVAssigns until DPVAssigns are enabled

This patch fixes commit 89aa3355, which added tests for
the removal of redundant DPVAssigns; unlike other cases where
adding tests for DPVAssigns before they are enabled is harmless,
these tests require them to be enabled, so must be deleted until
we enable them.

Fixes failures on llvm-new-debug-iterators buildbot:
https://lab.llvm.org/buildbot/#/builders/275/builds/3581