remove is_alternating() methods

Those were deprecated more than 3 years ago.

* spot/graph/graph.hh, spot/twa/twagraph.hh: Here.
* NEWS: Mention the change.

NEWS

@@ -111,6 +111,9 @@ New in spot 2.8.7.dev (not yet released)
     the input is Rabin-like or Streett-like, to_parity() should be at
     least as good as iar().
 
+  - The twa_graph::is_alternating() and digraph::is_alternating() methods,
+    deprecated in Spot 2.3.1 (2017-02-20), have been removed.
+
 New in spot 2.8.7 (2020-03-13)
 
   Bugs fixed:

spot/graph/graph.hh

@@ -1,5 +1,5 @@
 // -*- coding: utf-8 -*-
-// Copyright (C) 2014-2018 Laboratoire de Recherche et
+// Copyright (C) 2014-2018, 2020 Laboratoire de Recherche et
 // Développement de l'Epita.
 //
 // This file is part of Spot, a model checking library.
@@ -661,18 +661,6 @@ namespace spot
       return dests_.empty();
     }
 
-#ifndef SWIG
-    /// \brief Whether the automaton has universal branching
-    ///
-    /// The name of this function is confusing since non-deterministic
-    /// automata should be a subclass of alternating automata.
-    SPOT_DEPRECATED("use !is_existential() instead")
-    bool is_alternating() const
-    {
-      return !is_existential();
-    }
-#endif
-
     /// \brief Create a new states
     ///
     /// All arguments are forwarded to the State_Data constructor.

spot/twa/twagraph.hh

@@ -1,5 +1,6 @@
 // -*- coding: utf-8 -*-
-// Copyright (C) 2014-2019 Laboratoire de Recherche et Développement de l'Epita.
+// Copyright (C) 2014-2020 Laboratoire de Recherche et Développement
+// de l'Epita.
 //
 // This file is part of Spot, a model checking library.
 //
@@ -510,18 +511,6 @@ namespace spot
       return g_.is_existential();
     }
 
-#ifndef SWIG
-    /// \brief Whether the automaton has universal branching
-    ///
-    /// The name of this function is confusing since non-deterministic
-    /// automata should be a subclass of alternating automata.
-    SPOT_DEPRECATED("use !is_existential() instead")
-    bool is_alternating() const
-    {
-      return !is_existential();
-    }
-#endif
-
 #ifndef SWIG
     auto states() const
       SPOT_RETURN(g_.states());