Simplify ContainerNode::removeChildren
https://bugs.webkit.org/show_bug.cgi?id=115398

Reviewed by Andreas Kling.

Back port https://chromium.googlesource.com/chromium/blink/+/b981f01cd065236b5ff7e3a8446d15b384c1b557
by Andrei Bucur, which includes both this and his r148754.

* dom/ContainerNode.cpp:
(WebCore::ContainerNode::removeChildren):


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@149386 268f45cc-cd09-0410-ab3c-d52691b4dbfc