Remove WebSettings::setFixedPositionCreatesStackingContext.

After https://codereview.chromium.org/289973008/ lands,
there are no remaining call sites. And it's been a no-op
for over a month anyhow.

BUG=354688

Review URL: https://codereview.chromium.org/286263004

git-svn-id: svn://svn.chromium.org/blink/trunk@174969 bbb929c8-8fbe-4397-9dbb-9b2b20218538