Bug 1083591 followup. Remove not-yet-implemented stringifier.

dom/webidl/DOMMatrix.webidl

@@ -76,7 +76,6 @@ interface DOMMatrixReadOnly {
     DOMPoint                   transformPoint(optional DOMPointInit point);
     [Throws] Float32Array      toFloat32Array();
     [Throws] Float64Array      toFloat64Array();
-                               stringifier;
 };
 
 [Pref="layout.css.DOMMatrix.enabled",