Remove bool return value from IEditor::restoreState()

It's never used, and actually there wouldn't be any sensible thing to do
with it.

Change-Id: Id8a8df18c7db4b98e5abbc034240bb90dc1dcaa9
Reviewed-by: hjk <hjk@qt.io>

src/plugins/qmljseditor/qmljseditor.h

@@ -58,7 +58,7 @@ public:
     QmlJSEditorWidget();
 
     void finalizeInitialization() override;
-    bool restoreState(const QByteArray &state) override;
+    void restoreState(const QByteArray &state) override;
 
     QmlJSEditorDocument *qmlJsEditorDocument() const;