Update assert-pure.ql now that the /pure directory has been removed

[robertbrignull]

(Should be merged once all PRs for moving files out of the /pure directory have been merged)

Once the /pure directory has been removed by moving files elsewhere, we should also update update-pure.ql. It now only needs to check for files in /common, and we should rename it to not mention the word "pure" anywhere if at all possible.

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.

[aeisenberg]

Looks good.