js-on-sx: delete obj.key actually removes the key

js-delete-prop was setting value to js-undefined instead of
removing the key, so 'key' in obj remained true and proto-chain
lookup didn't fall through. Switched to dict-delete!.
Now delete Boolean.prototype.toString; Boolean.prototype.toString()
walks up to Object.prototype.toString and returns "[object Boolean]".
built-ins/Boolean: 21/27 → 22/27. conformance.sh: 148/148.

lib/js/runtime.sx

@@ -3517,7 +3517,7 @@
     (obj key)
     (cond
       ((dict? obj)
-        (begin (dict-set! obj (js-to-string key) js-undefined) true))
+        (begin (dict-delete! obj (js-to-string key)) true))
       (else true))))
 
 (define