Remove unused get_identity_relation.

Turns out we didnt need this as there is no unordered relations
managed by the oracle.

	* gimple-range-gori.cc (gori_compute::compute_operand1_range): Do
	not call get_identity_relation.
	(gori_compute::compute_operand2_range): Ditto.
	* value-relation.cc (get_identity_relation): Remove.
	* value-relation.h (get_identity_relation): Remove protyotype.