TextEditor: Reset mimetype after open() in all BaseTextEditors

This was already done in all major editors and should not harm
all others.

Change-Id: I7e19d285c4b85419a9a1f9afa961addecf682eaf
Reviewed-by: Eike Ziller <eike.ziller@digia.com>

src/plugins/qmljseditor/qmljseditor.h

@@ -145,7 +145,6 @@ class QmlJSEditor : public TextEditor::BaseTextEditor
 public:
     QmlJSEditor();
 
-    bool open(QString *errorString, const QString &fileName, const QString &realFileName);
     bool isDesignModePreferred() const;
 };