fix: don't start Xvfb in start.sh — sitespeed.io manages its own

Our start.sh was starting Xvfb on :99, then sitespeed.io tried to start
its own on :99, failed with exit code 1, and Chrome couldn't get a
working display. sitespeed.io is designed to manage Xvfb per test run.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

runner.js

@@ -31,7 +31,8 @@ export function runTest(job, onLine) {
       sitespeedArgs.push('--browsertime.chrome.args', 'disable-gpu');
     }
 
-    const env = { ...process.env, DISPLAY: process.env.DISPLAY || ':99' };
+    // Do not force DISPLAY — sitespeed.io starts and manages its own Xvfb
+    const env = { ...process.env };
 
     let child;