Rip out Settings::fixedPositionCreatesStackingContext

We always turn this setting on.

BUG=354688
R=ojan@chromium.org

Review URL: https://codereview.chromium.org/206463009

git-svn-id: svn://svn.chromium.org/blink/trunk@169714 bbb929c8-8fbe-4397-9dbb-9b2b20218538