TextInputWidget: Remove deprecated `DOMNodeInsertedIntoDocument` fallback
TextInputWidget: Remove deprecated `DOMNodeInsertedIntoDocument` fallback

It was supposedly a nice idea, but has never fully made it anywhere and
was standards deprecated.
According to caniuse only Opera 12.10 would be affected by this change,
it fully supported `DOMNodeInsertedIntoDocument` but not MutationObserver.
Anyways, it hasn't been part of modern support for a long while.

Change-Id: Ief78f208a68c52a24ffe0ca0979052612531c80f

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