Core: Remove mainwindow.{h,cpp}

Interface was moved to icore.h. The initial worry that lots of code
need changes was apparently unfounded.

Change-Id: I53a494d1ca914c7b1b16500c979029d06f5ab5f6
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/coreplugin/modemanager.cpp

@@ -10,7 +10,6 @@
 #include "fancytabwidget.h"
 #include "icore.h"
 #include "imode.h"
-#include "mainwindow.h"
 
 #include <extensionsystem/pluginmanager.h>