gdb/aarch64: Use default gdbarch methods where possible

Make use of the default gdbarch methods for gdbarch_dummy_id,
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:

	* gdb/aarch64-tdep.c (aarch64_dummy_id): Delete.
	(aarch64_unwind_pc): Delete.
	(aarch64_unwind_sp): Delete.
	(aarch64_gdbarch_init): Don't register deleted functions with
	gdbarch.