[llvm] [SystemZ] Removes string from the addressing mode classes (PR #215643)

[Ulrich Weigand via llvm-commits]

================

----------------
uweigand wrote:

These two should ideally be handled the same way as SystemZRegClass does it - maybe that macro needs to be refactored.   Also, it's weird to have this only for GR but not ADDR.

https://github.com/llvm/llvm-project/pull/215643