gdb/rl78: Use default gdbarch methods where possible

gdb/rl78: Use default gdbarch methods where possible

Make use of the default gdbarch method 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:

	* rl78-tdep.c (rl78_unwind_sp): Delete.
	(rl78_gdbarch_init): Don't register deleted function with gdbarch.