Remove --dotcucumber. No longer generate stepdefs.json.

This feature was added a long time ago to support autocomplete in
3rd party tools. The only tool that has used the generated stepdefs.json
file is Cucumber Pro, which recently abandoned it. (Cucumber Pro will
receive the same metadata over an API, via a custom formatter).
Merge pull request #737 from cucumber/remove-stepdefs-json

Remove --dotcucumber. No longer generate stepdefs.json.