Legacy platform objects should not override desc.[[Configurable]]

This is not implemented and WPT already tests for the opposite in
dom/collections/HTMLCollection-supported-property-names.html.

Fixes #669.

index.bs

@@ -13750,8 +13750,6 @@ Additionally, [=legacy platform objects=] have internal methods as defined in:
                     <emu-val>false</emu-val>.
                 1.  [=Invoke the named property setter=] on |O| with |P| and |Desc|.\[[Value]].
                 1.  Return <emu-val>true</emu-val>.
-    1.  If |O| does not implement an [=interface=] with the [{{Global}}] [=extended attribute=],
-        then set |Desc|.\[[Configurable]] to <emu-val>true</emu-val>.
     1.  Return [=!=] <a abstract-op>OrdinaryDefineOwnProperty</a>(|O|, |P|, |Desc|).
 </div>