gdb/s12z: Use default gdbarch methods where possible

gdb/s12z: Use default gdbarch methods where possible

Make use of the default gdbarch methods for gdbarch_unwind_pc, and
gdbarch_unwind_sp where possible.

I have not tested this change but, by inspecting the code, I believe
the default methods are equivalent to the code being deleted.

gdb/ChangeLog:

	* s12z-tdep.c (s12z_unwind_pc): Delete.
	(s12z_unwind_sp): Delete.
	(s12z_gdbarch_init): Don't register deleted functions with
	gdbarch.