remove useless forward declaration

* spot/ta/taexplicit.hh,
spot/twaalgos/compsusp.hh,
spot/twaalgos/isunamb.hh,
spot/twaalgos/word.hh: Here.

spot/ta/taexplicit.hh

 // -*- coding: utf-8 -*-
-// Copyright (C) 2010, 2011, 2012, 2013, 2014, 2015, 2016 Laboratoire
+// Copyright (C) 2010, 2011, 2012, 2013, 2014, 2015, 2016, 2018 Laboratoire
 // de Recherche et Développement de l'Epita (LRDE).
 //
 // This file is part of Spot, a model checking library.
@@ -32,7 +32,6 @@ namespace spot
 {
   // Forward declarations.  See below.
   class state_ta_explicit;
-  class ta_explicit_succ_iterator;
   class ta_explicit;
 
   /// Explicit representation of a spot::ta.

spot/twaalgos/compsusp.hh

 // -*- coding: utf-8 -*-
-// Copyright (C) 2012, 2013, 2014, 2015 Laboratoire de Recherche et
+// Copyright (C) 2012, 2013, 2014, 2015, 2018 Laboratoire de Recherche et
 // Développement de l'Epita (LRDE).
 //
 // This file is part of Spot, a model checking library.
@@ -24,8 +24,6 @@
 
 namespace spot
 {
-  class bdd_dict;
-
   /// \brief Compositional translation algorithm with resetable
   /// suspension.
   ///

spot/twaalgos/isunamb.hh

 // -*- coding: utf-8 -*-
-// Copyright (C) 2013, 2015 Laboratoire de Recherche et Developpement
+// Copyright (C) 2013, 2015, 2018 Laboratoire de Recherche et Developpement
 // de l'Epita (LRDE).
 //
 // This file is part of Spot, a model checking library.
@@ -23,8 +23,6 @@
 
 namespace spot
 {
-  class tgba;
-
   /// \addtogroup twa_misc
   /// @{
 

spot/twaalgos/word.hh

 // -*- coding: utf-8 -*-
-// Copyright (C) 2013, 2014, 2015, 2016 Laboratoire de Recherche et
+// Copyright (C) 2013, 2014, 2015, 2016, 2018 Laboratoire de Recherche et
 // Développement de l'Epita (LRDE).
 //
 // This file is part of Spot, a model checking library.
@@ -23,8 +23,6 @@
 
 namespace spot
 {
-  class bdd_dict;
-
   /// \brief An infinite word stored as a lasso.
   ///
   /// This is not exactly a word in the traditional sense because we