Remove CSSParserValue::createCSSValue and unused callers

The function CSSParserValue::createCSSValue is only actually used by
calc parsing, although we can more easily do the same thing there in
much less code.

BUG=404023

Review URL: https://codereview.chromium.org/470903003

git-svn-id: svn://svn.chromium.org/blink/trunk@180325 bbb929c8-8fbe-4397-9dbb-9b2b20218538