Remove registerSaveDescription from GCStackMap

[0xdaryl]

With the removal of shrink wrapping (see eclipse-omr/omr#2107) the register save description encoded in the GC stack map (and therefore encoded in a method's metadata) is no longer required. Carefully remove it.

This is the only consumer of the RSD from the Instruction class, which is not set anywhere and is slated for removal in #7203.

Note, the Register Save Description in the CodeGenerator is still required.