Revert "Add a release assert in Element's constructor to track down a crasher"

This reverts r164442.

The RELEASE_ASSERT never fired in production, so we might as well remove it.

R=abarth@chromium.org
BUG=312410

Review URL: https://codereview.chromium.org/145263006

git-svn-id: svn://svn.chromium.org/blink/trunk@165779 bbb929c8-8fbe-4397-9dbb-9b2b20218538