Delete dangling declaration

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src/libstore/include/nix/store/build/derivation-building-goal.hh

@@ -174,8 +174,6 @@ struct DerivationBuildingGoal : public Goal
 
     void appendLogTailErrorMsg(std::string & msg);
 
-    StorePathSet exportReferences(const StorePathSet & storePaths);
-
     JobCategory jobCategory() const override
     {
         return JobCategory::Build;