Delete User::dropHungOffUses and move it in to ~User which is the only caller. NFC.

Delete User::dropHungOffUses and move it in to ~User which is the only caller. NFC.

Now that the subclasses which care about hung off uses let ~User clean it up,
there's no need for a separate method.  Just inline it to ~User and delete it.

Reviewed by Duncan Exon Smith.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@239491 91177308-0d34-0410-b5e6-96231b3b80d8