Remove support for CompositingReasonPositionSticky

Our support for position: sticky isn't even in the experimental feature set. We
shouldn't be burning time during every frame checking whether any RenderLayers
have sticky position.

R=esprehn@chromium.org

Review URL: https://codereview.chromium.org/328263002

git-svn-id: svn://svn.chromium.org/blink/trunk@175992 bbb929c8-8fbe-4397-9dbb-9b2b20218538