gdb/hppa: Use default gdbarch method default_dummy_id.

Make use of the default gdbarch method for gdbarch_dummy_id.

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

gdb/ChangeLog:

	* gdb/hppa-tdep.c (hppa_dummy_id): Delete.
	(hppa_gdbarch_init): Don't register deleted functions with
	gdbarch.