[BREAKING CHANGE] TitledElement.php: Remove $element::$title fallback
[BREAKING CHANGE] TitledElement.php: Remove $element::$title fallback

This feature has probably never been used, and barely anyone knew about
it. Because it uses the same name as the $title property of
TitledElement, it would be problematic to support when mixins are
converted to use proper PHP traits.

Change-Id: I654e83e07758f01573cb1610ec1b0c830065b1d3

wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui