EquationTags: remove unused function

src/EquationTags.hh

 /*
- * Copyright © 2020-2022 Dynare Team
+ * Copyright © 2020-2023 Dynare Team
  *
  * This file is part of Dynare.
  *
@@ -125,13 +125,6 @@ public:
     return exists(eqn) && eqn_tags.at(eqn).contains(key);
   }
 
-  //! Returns true if equation tag with key and value exists for a given equation
-  bool
-  exists(const int eqn, const string &key, const string &value) const
-  {
-    return exists(eqn, key) && eqn_tags.at(eqn).at(key) == value;
-  }
-
   //! Various functions to write equation tags
   void writeCheckSumInfo(ostream &output) const;
   void writeOutput(ostream &output) const;