remove support for labelled statements

[rafaelw]

this will be breaking change when we do it.

[rafaelw]

use of tokenList and styleObject filters are now preferred. e.g.

https://github.com/Polymer/polymer-expressions/blob/master/tests/tests.js#L178

and

https://github.com/Polymer/polymer-expressions/blob/master/tests/tests.js#L116

removed here: 43688ea