gdb/nios2: Use default gdbarch methods where possible

gdb/nios2: Use default gdbarch methods where possible

Make use of the default gdbarch methods for gdbarch_dummy_id, 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:

	* nios2-tdep.c (nios2_dummy_id): Delete.
	(nios2_unwind_sp): Delete.
	(nios2_gdbarch_init): Don't register deleted functions with
	gdbarch.