Core: Clean up some ICore::instance() uses

Change-Id: I888b30094ed75818ee3af58c94ebf8468afa5eb9
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>

src/plugins/coreplugin/icore.cpp

@@ -287,23 +287,18 @@
 
 #include <utils/hostosinfo.h>
 
-#include <QDir>
 #include <QCoreApplication>
 #include <QDebug>
-
+#include <QDir>
 #include <QStatusBar>
 
+using namespace Core::Internal;
+
 namespace Core {
 
 // The Core Singleton
 static ICore *m_instance = 0;
-
-namespace Internal {
 static MainWindow *m_mainwindow;
-} // namespace Internal
-
-using namespace Core::Internal;
-
 
 ICore *ICore::instance()
 {