Remove dead RenderLayerCompositor::setCompositingParent

This code is dead, we always pass a null parentLayer which causes us to
early return. I think we were supposed to call
childLayer->compositedLayerMapping()->childForSuperlayers()->removeFromParent()
in the original design, but since r44876 [1] we always early return when the
parentLayer is null, and that patch was back in 2009 so clearly the code
is not needed.

[1] http://src.chromium.org/viewvc/blink?revision=44876&view=revision

R=abarth@chromium.org

Review URL: https://codereview.chromium.org/279643002

git-svn-id: svn://svn.chromium.org/blink/trunk@173705 bbb929c8-8fbe-4397-9dbb-9b2b20218538