Remove wrong commented out code from RenderLayer::updateLayerPositionsAfterScroll

This comment is about ASSERTing the consistency of state that no long exists.
Also, remove m_canSkipRepaintRectsUpdateOnScroll because it's just
renderer()->isTableCell().

R=esprehn@chromium.org

Review URL: https://codereview.chromium.org/356123004

git-svn-id: svn://svn.chromium.org/blink/trunk@177129 bbb929c8-8fbe-4397-9dbb-9b2b20218538