Remove Buildkite references

[KipHamiltons]

$ grep -Iril 'buildkite' ./src

./src/book/02-system/01-foundations/assets/ci_setup.sh
./src/book/03-guides/04-general/02-git.mdx
./src/book/03-guides/04-general/04-glossary.mdx

The glossary references are proper, post CI page changes which are pending. The git page and the ci_setup page should be updated to remove those.

[LachlanCourt]

We can probably just remove that script altogether

[JosephusPaye]

ci_setup.sh was written to be run on new computers to set them up as a build agent. If we don't have our own runners with GitHub Actions, it can be removed. If we do it can be updated to setup a GitHub Actions runner instead.