refine: remove unused on helper from website theme JS

[prql-bot]

main.js is a single IIFE with no exports, so its on helper is only reachable from inside that closure — and nothing there calls it. The one listener the file actually wires up is the back-to-top button, which goes through onscroll.

With on gone, nothing passes all = true any more, so select's all branch (return [...document.querySelectorAll(el)]) is unreachable too — the second commit drops it, leaving select as a one-line document.querySelector wrapper. The sole call site, select(".back-to-top"), already took the default all = false. select and onscroll are both still used and stay.

Removing it leaves behaviour identical.

Found during the nightly survey of web/website/themes/prql-theme/static/main.js. No test — the site has no JS test suite, and this is a pure removal of unreachable code.

[prql-bot]

Verified: main.js is a self-contained IIFE loaded on its own from baseof.html (<script src="{{ "/main.js" | relURL }}">), on was never assigned to a global, and no layout or other theme script references it — the removal is behaviour-preserving.

One follow-on, outside this diff: on was the only caller that ever passed all = true, so the all branch inside select — return [...document.querySelectorAll(el)] — is now unreachable for the same reason. The single remaining call is select(".back-to-top"), which takes the default all = false. If the intent is to clear the dead code rather than keep select general-purpose, that branch belongs in this PR too; happy to push the follow-up commit either way.

[prql-bot]

Took the follow-on — the PR's whole point is clearing dead code, so leaving a branch that can no longer be reached would be half a job. 699f611 drops the all parameter and the querySelectorAll branch, so select is now const select = (el) => document.querySelector(el.trim());. The one call site, select(".back-to-top"), was already on the default path, and node --check plus prettier --check both pass on the result. PR description updated to cover both commits.