Remove ENABLE_CSS3_TEXT flag which is not used anymore.

text-underline-position, which was the last property behind the flag,
is changed to use runtime flag.

Review URL: https://codereview.chromium.org/54683003

git-svn-id: svn://svn.chromium.org/blink/trunk@161240 bbb929c8-8fbe-4397-9dbb-9b2b20218538