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/perforce/perforceplugin.cpp

@@ -1567,6 +1567,4 @@ void PerforcePlugin::testLogResolving()
 } // namespace Internal
 } // namespace Perforce
 
-Q_EXPORT_PLUGIN(Perforce::Internal::PerforcePlugin)
-
 #include "perforceplugin.moc"