starmath: Drop unnecessary forward declaration

Change-Id: I0f8893f6aa34a9738aad1e1a96f779bc77672f68
Reviewed-on: https://gerrit.libreoffice.org/23848
Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: David Tardon <dtardon@redhat.com>

starmath/inc/dialog.hxx

@@ -344,8 +344,6 @@ public:
 };
 
 
-class SmSymDefineDialog;
-
 class SmSymbolDialog : public ModalDialog
 {
     VclPtr<ListBox>         m_pSymbolSets;

starmath/inc/document.hxx

@@ -42,7 +42,6 @@
 #include "smdllapi.hxx"
 
 class SmNode;
-class SfxMenuBarManager;
 class SfxPrinter;
 class Printer;
 class SmCursor;

starmath/inc/smmod.hxx

@@ -34,7 +34,6 @@
 #include <memory>
 
 class SfxObjectFactory;
-class SmModule;
 class SmSymbolManager;
 class SmMathConfig;
 

starmath/inc/view.hxx

@@ -36,7 +36,6 @@
 #include "edit.hxx"
 #include "node.hxx"
 
-class Menu;
 class DataChangedEvent;
 class SmDocShell;
 class SmViewShell;