CorePlugin: Clean forward declaration

Change-Id: I37c381bc26a29acae1bfe0e49f2772b4e92cf046
Reviewed-by: Orgad Shaneh <orgads@gmail.com>

src/plugins/coreplugin/icore.h

@@ -44,9 +44,7 @@ namespace Core {
 class IWizardFactory;
 class Context;
 class IContext;
-class ProgressManager;
 class SettingsDatabase;
-class VcsManager;
 
 namespace Internal { class MainWindow; }