docs(student): drop blur warning from join disclaimer

A multi-hour lecture has questions spread over the whole period; we can't
reasonably expect students to keep their devices focused for that long.
Blur events still get logged (no code change) so we have signal for
post-hoc analysis if the histogram is striking, but we don't promise it
as enforcement, and we don't make students self-monitor against an
unrealistic standard.

static/quiz.js

@@ -178,7 +178,6 @@ function renderJoin(error = null) {
           <li>If you see <em>"This student ID is already in use"</em>, <b>do not retry</b> — every attempt is recorded. Tell the instructor and they will reset your slot.</li>
           <li>Asking the instructor to reset your slot will set <b>all already-closed questions to 0</b> (status: missed). This is permanent and applies whether or not the slot was actually hijacked.</li>
           <li>Do not clear your cookies during the quiz. Clearing them locks you out and recovery requires a reset (same penalty as above).</li>
-          <li>Tab and window switches during a live question are logged for the instructor.</li>
           <li>This portal is for in-lecture engagement; attendance is taken on paper.</li>
         </ul>
       </details>