remove yaml comment syntax highlighting - it's too aggressive

This PR removes the yaml file formats syntax highlighting for line comments. It's too aggressive and matches valid yaml file contents (like urls).

![screen shot 2014-02-22 at 9 53 31 am](https://f.cloud.github.com/assets/1269969/2238366/902bffba-9bea-11e3-9d3b-d3df17048bba.png)