gdb/arc: 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.  The
only difference is that the old ARC specific methods had additional
debugging, however, this was discussed on the mailing list[1] and it
was agreed these methods could be removed.

[1] https://sourceware.org/ml/gdb-patches/2018-12/msg00386.html

gdb/ChangeLog:

	* gdb/arc-tdep.c (arc_dummy_id): Delete.
	(arc_unwind_pc): Delete.
	(arc_unwind_sp): Delete.
	(arc_gdbarch_init): Don't register deleted functions with gdbarch.