Revert "Sky: When an iframe element is removed, delete its mojo View."

This reverts commit 45991a3752e6a6298901eba36e7bf4cdec4d263c.

It was causing flaky crashes on the bots.

TBR=esprehn@chromium.org
BUG=434555

Review URL: https://codereview.chromium.org/740773002

engine/src/flutter/tests/lowlevel/iframe.sky

@@ -1,12 +1,6 @@
 <sky>
     <import src="../resources/dump-as-render-tree.sky" />
-    <iframe id="removeMe" src="text.sky">Should not render</iframe>
     <div>This is an <iframe style="border-style: solid; border-width: 20px" src="abarth.sky">iframe</iframe> element.</div>
     <div style="height: 200px">Placeholder.</div>
     <div>Small iframe should render <iframe style="height: 40px" src="abarth.sky">iframe</iframe> correctly.</div>
-
-    <script>
-        var removeMe = document.getElementById("removeMe");
-        removeMe.parentNode.removeChild(removeMe);
-    </script>
 </sky>