Remove unused CSSPrimitiveValue::CSS_PARSER_{OPERATOR,INTEGER,IDENTIFIER}

CSS_PARSER_INTEGER is only used as an alias for CSS_NUMBER, and the
enums CSS_PARSER_OPERATOR and CSS_PARSER_IDENTIFIER are both unused.

Review URL: https://codereview.chromium.org/476693002

git-svn-id: svn://svn.chromium.org/blink/trunk@180334 bbb929c8-8fbe-4397-9dbb-9b2b20218538