Remove JSON formatter

[aslakhellesoy]

As described in #1249 the plan is to deprecate the json formatter in version 7. In version 8 we should remove it.

After the removal people can still generate JSON reports with the standalone json formatter. There are prebuilt executables that will convert output from the protobuf formatter to JSON. This is intended to be used only by people who have to use tools that still only support the old JSON format.

[gkushang]

I disagree with this change as it will break a lot of existing modules. I strongly feel cucumber should keep in-built support for JSON formatted as there are lot of existing Report Dashboards, HTML Reporters are dependent on it.

The link https://github.com/cucumber/cucumber/tree/master/json-formatter is broken.