messages: Clean up unsued message prior to release

[mpkorstanje]

The message protocol still contains a number of messages that are related to Cucumber Engine that currently go unused. Separating these out now, will:

• Make it easier to evolve the protocol later
• Make it easier for people to understand the current implementation
• Remove a large amount of effectively unused code and documentation.

Once they're included in a release from Cucumber this will all become harder to reason about.

[aslakhellesoy]

I agree. I will make a PR to remove messages that aren't referenced by any of the producers or consumers in the monorepo:

• Producers: gherkin, fake-cucumber and json-to-features (Translate legacy Json to messages #938)
• Consumers: *-formatter and react

This means the messages that are only referenced cucumber-engine would have to be moved elsewhere. But that project seems to have fallen by the wayside now. Thoughts @charlierudolph?

[charlierudolph]

I'm good with removing them. I can pull them from history if I need them for cucumber-engine but think that may not be used with the current evolution. Was fun to write and can maybe be used a bit in cucumber-go

[aslakhellesoy]

Fixed by #962