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