TextEditor: update the infobar after setting up the highlighter

removes the need of public function and member carrying the
information whether a highlight definition was found.

Change-Id: I8a0f24c9b376c01246116b502f5bbc06b3c65d21
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/plugins/texteditor/texteditor.h

@@ -332,7 +332,6 @@ public:
 
     // the blocks list must be sorted
     void setIfdefedOutBlocks(const QList<BlockRange> &blocks);
-    bool isMissingSyntaxDefinition() const;
 
     enum Side { Left, Right };
     QAction *insertExtraToolBarWidget(Side side, QWidget *widget);