ExtensionSystem: Remove Qt4 support

Remove usage of the Q_EXPORT_PLUGIN macros, which do not exist in Qt 5.

Change-Id: I678c3cf10b9c5d5c1b9f252b0ecd1c97dc810a47
Reviewed-by: hjk <hjk121@nokiamail.com>
Reviewed-by: Christian Stenger <christian.stenger@digia.com>

src/plugins/classview/classviewplugin.cpp

@@ -62,5 +62,3 @@ bool Plugin::initialize(const QStringList &arguments, QString *errorMessage)
 
 } // namespace Internal
 } // namespace ClassView
-
-Q_EXPORT_PLUGIN(ClassView::Internal::Plugin)