This apparently gradually fell out of use after the more general
Token::IsIdentifer was introduced, and whoever left last forgot to
turn out the lights.

The only remaining use was in an assertion in DeclareLabel, but
meanwhile DeclareLabel is only called if IsIdentifier. I added re-formulated assertions, just in case.

R=verwaest@chromium.org
BUG=v8:4947

Review-Url: https://codereview.chromium.org/2549493002
Cr-Commit-Position: refs/heads/master@{#41452}