Deconstruct old CI pipeline
Moving forward to [our new shiny CI](https://ci.kaitai.io/), we don't really need [our old monolithic sequential pipeline](https://github.com/kaitai-io/kaitai_struct_tests/blob/master/ci-all).

It already gives wrong impressions that we're relying on it, and it already slows down new pipeline doing lots of stuff in vain.

The plan:

- [x] Remove per-language support from [main project's .travis.yml](https://github.com/kaitai-io/kaitai_struct/blob/master/.travis.yml)
- [x] Remove [ci-all](https://github.com/kaitai-io/kaitai_struct_tests/blob/master/ci-all)-related machinery from tests repo
- [x] Remove [aggregated HTML page renderer](https://github.com/kaitai-io/kaitai_struct_tests/blob/master/aggregate/aggregate)
- [x] Update developer docs to show overall scheme of new CI

Cc @13rac1