[SystemZ] Remove bogus isAsmParserOnly

Marking instructions as isAsmParserOnly stops them from being disassembled.
However, in cases where separate asm and codegen versions exist, we actually
want to disassemble to the asm ones.

No functional change intended.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@181772 91177308-0d34-0410-b5e6-96231b3b80d8