Remove CSSParserMode::needsSiteSpecificQuirks

The boolean is unused after https://codereview.chromium.org/63383003
but this removal was forgotten in the original change.

Review URL: https://codereview.chromium.org/70433004

git-svn-id: svn://svn.chromium.org/blink/trunk@161881 bbb929c8-8fbe-4397-9dbb-9b2b20218538