Remove web_steps.rb

The cucumber-rails authors removed `web_steps.rb` in version 1.1.0 (cucumber/cucumber-rails#174) because they believed it led to misuse of Cucumber. (See [The training wheels came off](http://aslakhellesoy.com/post/11055981222/the-training-wheels-came-off) for more background.)

I propose doing the same for cucumber-sinatra.

I removed the related template files and updated the generator. `features/step_definitions` is the default directory for step definitions, so rather than omit that directory it makes sense to create an empty `features/step_definitions` to help new Cukers get started. We could also create a dummy features file and corresponding step definition file, but that might require additional thought to do the right thing.

It doesn't look like there is any automated testing to verify my changes, but I did successfully run `cucumber-sinatra` with the updated version. Let me know whether I can help with any other testing. I also did not bump the version, since I don't know how you'd want to handle that.