Remove unnecessary SVGUseElement::finishParsingChildren

After [1], this function is no longer needed.

[1] https://src.chromium.org/viewvc/blink?view=rev&revision=174662

Review URL: https://codereview.chromium.org/299293002

git-svn-id: svn://svn.chromium.org/blink/trunk@174730 bbb929c8-8fbe-4397-9dbb-9b2b20218538