gdb/or1k: Use default gdbarch methods where possible

gdb/or1k: Use default gdbarch methods where possible

Make use of the default gdbarch method gdbarch_dummy_id 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.

This commit leaves or1k_unwind_sp and or1k_unwind_pc in place.  These
functions do match the default methods except that they add additional
debugging code.  In order to preserve the debug I have left these
functions unchanged.

gdb/ChangeLog:

	* or1k-tdep.c (or1k_dummy_id): Delete.
	(or1k_gdbarch_init): Don't register deleted function with gdbarch.