[SystemZ] Remove the ManipulatesSP flag from backend (NFC).

This flag was set in the presence of stacksave/stackrestore in order to force
a frame pointer.

This should however not be needed per the comment in MachineFrameInfo.h
stating that a a variable sized object "...is the sole condition which
prevents frame pointer elimination", and experiments have also shown that
there seems to be no effect whatsoever on code generation with ManipulatesSP.

Review: Ulrich Weigand