Remove repo contents after migration

[adityasharad]

The contents of this repository have been migrated to the github/codeql repository, specifically https://github.com/github/codeql/tree/main/go.

See https://github.com/github/codeql-go/issues/741 for details.

Remove the contents of this repo, keeping only licensing and repo docs, so that users who clone both the codeql and codeql-go repos do not end up with conflicting source files, particularly CodeQL packs.

Update the README with a similar notice.

[aeisenberg]

Can you tag the final contents of this repo (current tip of main most likely) and include a link to that tag in the README?

[aeisenberg]

Also, I think we need to disable the LGTM checks.