IR: Remove reference to ENABLE_MDNODE_UNIQUING

Apparently `MDNode` uniquing used to be optional.  I suppose the
configure flag must have disappeared at some point.  Change the test so
it actually tests uniquing, and remove the check for
`ENABLE_MDNODE_UNIQUING`.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223617 91177308-0d34-0410-b5e6-96231b3b80d8