[LVI] Revert the remainder of "r218231 - Add two thresholds lvi-overdefined-BB-threshold and lvi-overdefined-threshold"

Some of r218231 was reverted with the code that used it in r218971, but not all
of it. This removes the rest (which is now dead).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219469 91177308-0d34-0410-b5e6-96231b3b80d8