Remove defensive code now that the stack trace leading to it is gone.

crrev.com/325063003 deleted RenderLayerCompositor::layerWillBeRemoved, which
was a critical part of the stack trace that led to this code crashing.

BUG=370410

Review URL: https://codereview.chromium.org/358473005

git-svn-id: svn://svn.chromium.org/blink/trunk@176881 bbb929c8-8fbe-4397-9dbb-9b2b20218538