twa: call unregister_all_my_variables()

Fixes #129.

* spot/twa/twa.cc (~twa): call unregister_all_my_variables()
* spot/twa/twagraph.hh, spot/twa/twaproduct.cc,
spot/twaalgos/stutter.cc: Simplify.
* NEWS: Mention the change.

NEWS

@@ -2,7 +2,7 @@ New in spot 1.99.6a (not yet released)
 
   Command-line tools:
 
-  * BACKWARD INCOMPATIBILE CHANGE: All tools that output automata now
+  * BACKWARD INCOMPATIBLE CHANGE: All tools that output automata now
     use the HOA format by default instead of the GraphViz output.
     This makes it easier to pipe several commands.
 
@@ -41,6 +41,10 @@ New in spot 1.99.6a (not yet released)
     renamed to spot/ltsmin/, so that installed and non-installed
     directories can be used similarly.
 
+  * twa::~twa() is now calling
+      get_dict()->unregister_all_my_variable(this);
+    so this does not need to be done in any subclass.
+
   * is_inherently_weak_automaton() is a new function, and
     check_strength() has been modified to also check inherently weak
     automata.

spot/twa/twa.cc

@@ -43,6 +43,7 @@ namespace spot
       last_support_conditions_input_->destroy();
     delete iter_cache_;
     release_named_properties();
+    get_dict()->unregister_all_my_variables(this);
   }
 
   bdd

spot/twa/twagraph.hh

@@ -202,7 +202,6 @@ namespace spot
 
     virtual ~twa_graph()
     {
-      get_dict()->unregister_all_my_variables(this);
       // Prevent this state from being destroyed by ~twa(),
       // as the state will be destroyed when g_ is destroyed.
       last_support_conditions_input_ = nullptr;

spot/twa/twaproduct.cc

@@ -315,7 +315,6 @@ namespace spot
 
   twa_product::~twa_product()
   {
-    get_dict()->unregister_all_my_variables(this);
     // Prevent these states from being destroyed by ~tgba(): they
     // will be destroyed before when the pool is destructed.
     if (last_support_conditions_input_)

spot/twaalgos/stutter.cc

@@ -215,11 +215,6 @@ namespace spot
 	acc_.set_generalized_buchi();
       }
 
-      virtual ~tgbasl()
-      {
-	get_dict()->unregister_all_my_variables(this);
-      }
-
       virtual const state* get_init_state() const override
       {
 	return new state_tgbasl(a_->get_init_state(), bddfalse);