fix(telemetry): drop events:["startup"] from Telemetry experiment API

events:["startup"] requires the ExtensionAPI to implement onStartup(); the
Telemetry API doesn't, so declaring it threw "api.onStartup is not a function"
and aborted the add-on's bootstrap startup. It was also unnecessary — an
event-only API (cf. AccountHub) is available at background startup without it.
The original "browser.Telemetry undefined" was a stale built-in packaging issue,
not a startup-timing one. The background guard added earlier is retained.

Refs #952

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: aaspinwall

packages/addon/public/manifest.json

@@ -136,10 +136,7 @@
             "Telemetry"
           ]
         ],
-        "script": "api/Telemetry/implementation.js",
-        "events": [
-          "startup"
-        ]
+        "script": "api/Telemetry/implementation.js"
       }
     }
   },