modeleditor: remove unused forward declaration

Change-Id: I7c9c6856318145214ce9e76feeb721abed3b4e61
Reviewed-by: Orgad Shaneh <orgads@gmail.com>

src/plugins/modeleditor/actionhandler.h

@@ -37,7 +37,6 @@ class QAction;
 QT_END_NAMESPACE
 
 namespace Core {
-class ActionContainer;
 class Command;
 }
 

src/plugins/modeleditor/diagramsviewmanager.h

@@ -31,8 +31,6 @@
 #include <QHash>
 
 namespace qmt {
-class Uid;
-class DiagramView;
 class MDiagram;
 }
 

src/plugins/modeleditor/modeleditor.h

@@ -42,7 +42,6 @@ class MPackage;
 class MDiagram;
 class DElement;
 class DContainer;
-class DocumentController;
 }
 
 namespace ModelEditor {
@@ -50,7 +49,6 @@ namespace Internal {
 
 class UiController;
 class ActionHandler;
-class DiagramsViewManager;
 
 enum class SelectedArea {
     Nothing,