Remove bitbucket.org/zombiezen/yaml package.

The package has been last updated in early 2013. The description says it implements a YAML 1.2 parser, but as we've learned at https://github.com/shurcooL/markdownfmt/issues/11#issuecomment-54249637, it has high level parsing functionality completely unimplemented and marked as TODO.

The quality standard says:

> curated list for high-quality, actively maintained Go packages

> - Functional
> - Actively maintained (even if that just means acknowledging open issues when they arise)
> - Stable, or progressing toward stable

This doesn't seem to meet that criteria.

This is simply a suggestion to remove it, please discuss and act as appropriate.