gdb/vax: Use default gdbarch methods where possible

gdb/vax: Use default gdbarch methods where possible

Make use of the default gdbarch method gdbarch_unwind_pc 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:

	* vax-tdep.c (vax_unwind_pc): Delete.
	(vax_gdbarch_init): Don't register deleted function with gdbarch.