Remove apparently unneeded QtPlugin includes

Change-Id: I511c0318a72dbf2a92e4749dee02eabfc9826139
Reviewed-by: Alessandro Portale <alessandro.portale@qt.io>
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/perforce/perforceplugin.cpp

@@ -68,7 +68,6 @@
 #include <QMessageBox>
 #include <QSettings>
 #include <QTextCodec>
-#include <QtPlugin>
 
 using namespace Core;
 using namespace Utils;