gdb/alpha: Use default gdbarch methods where possible

Make use of the default gdbarch methods for gdbarch_dummy_id, and
gdbarch_unwind_pc.

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/alpha-tdep.c (alpha_dummy_id): Delete.
	(alpha_unwind_pc): Delete.
	(alpha_gdbarch_init): Don't register deleted functions with
	gdbarch.