Minor cleanups to RenderLayerCompositor

This CL removes an unused function and deletes an unused local variable.

TBR=eseidel@chromium.org

Review URL: https://codereview.chromium.org/177103003

git-svn-id: svn://svn.chromium.org/blink/trunk@167686 bbb929c8-8fbe-4397-9dbb-9b2b20218538