core: remove usages of Core::ICore::instance()->modeManager()

The function is still there but should be remove in my opinion.

Change-Id: I739fe942e74f459faccb80b5bcb8a5721c6bb9bf
Reviewed-on: http://codereview.qt.nokia.com/4200
Reviewed-by: hjk <qthjk@ovi.com>

src/plugins/coreplugin/modemanager.cpp

@@ -60,6 +60,15 @@
 
 namespace Core {
 
+/*!
+    \class Core::ModeManager
+
+    The mode manager handles everything related to the instances of IMode
+    that were added to the plugin manager's object pool as well as their
+    buttons and the tool bar with the round buttons in the lower left
+    corner of Qt Creator.
+*/
+
 struct ModeManagerPrivate
 {
     explicit ModeManagerPrivate(Internal::MainWindow *mainWindow,