c++: Remove ifdefed code

The only reason I chose to use DECL_UID on this hash table was to make
it stable against ASLR and perturbations due to other allocations.
It's not required for correctness, as the comment mentions the
equality fn uses pointer identity.

	gcc/cp/
	* module.cc (struct duplicate_hash): Remove.
	(duplicate_hash_map): Adjust.