Remove non-existent unload event handler from webagg image object - #32402

Summary

The webagg frontend (lib/matplotlib/backends/web_backend/js/mpl.js) assigned an onunload handler to the rendering Image element that closed the websocket. No unload event exists on HTMLImageElement or any of its ancestors (HTMLElement, Element, Node) — per the MDN docs it only exists on the global Window interface. The assignment was a plain expando property that no browser ever invoked, so the handler has been dead code since it was introduced.

Removing it outright (rather than migrating to pagehide/visibilitychange) is behavior-neutral because the real cleanup paths are untouched:

• On page teardown the browser closes all open WebSocket connections; the tornado side already handles that via WebSocketHandler.on_close → FigureManagerWebAgg.remove_web_socket (backend_webagg.py).
• For nbagg, the comm is closed through Jupyter's own comm lifecycle (comm.on_close in backend_nbagg.py), independent of this handler.

Testing

• node --check js/mpl.js — syntax OK
• grep -c onunload js/*.js — 0 occurrences remain
• Diff is a pure 4-line deletion; no other references to the handler exist in the repo
• The JS is shipped unbundled (meson install_subdir('js'), served via StaticFileHandler), so no build step applies

Fixes #32393