[SystemZ] Revert r191661: Add definitions of LFH and STFH

For some reason, adding definitions for these load and store
instructions changed whether some of the build bots matched
comparisons as signed or unsigned.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@191663 91177308-0d34-0410-b5e6-96231b3b80d8