Revert "Add -mcpu=z10 to SystemZ tests."

This reverts commit r197466.

The MachineCSE fix that required the -mcpu flag has been disabled
until more work can be done to fix downstream issues. Adding -mcpu
wasn't the right workaround anyway.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@197624 91177308-0d34-0410-b5e6-96231b3b80d8