Remove or fix dead code / unused exports

[robertbrignull]

See #2631 for the dead code CI check. This PR is the result of going through the codebase fixing all of the cases found by that check. Where there is code deleted I've done each item as a separate commit, and then at the end I've done one commit for all the cases of things that were unnecessarily-exported but still used within their file.

I haven't tested all features yet to make sure everything still works, but from what I could tell as I was going through things there's nothing that's likely to not be caught at compile time, so I'm fairly confident this is all ok so long as it compiles.

Checklist

• CHANGELOG.md has been updated to incorporate all user visible changes made by this pull request.
• Issues have been created for any UI or other user-facing changes made by this pull request.
• [Maintainers only] If this pull request makes user-facing changes that require documentation changes, open a corresponding docs pull request in the github/codeql repo and add the ready-for-doc-review label there.

[charisk]

Thanks for cleaning up! 🧹

[robertbrignull]

It seems that extensions/ql-vscode/test/__mocks__/styleMock.ts and extensions/ql-vscode/test/__mocks__/fileMock.ts are in fact needed. I missed the references to them originally. I've added those back in (by removing the commits that deleted them) and I'll mark them as allowed in #2631

[robertbrignull]

Annoying I rebased on the wrong base commit originally, but https://github.com/github/vscode-codeql/compare/7f363723f9c0e4c2d4703172d2f5497a00a6e1b3..8e8e0faa9e4e0284d0ea5ec2be9dfbb06f8515ca shows the change from where this PR was at before.