gdb/rs6000: Use default gdbarch methods where possible

gdb/rs6000: Use default gdbarch methods where possible

Make use of the default gdbarch methods for gdbarch_dummy_id,
gdbarch_unwind_pc 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:

	* rs6000-tdep.c (rs6000_unwind_pc): Delete.
	(rs6000_dummy_id): Delete.
	(rs6000_gdbarch_init): Don't register deleted functions with
	gdbarch.