Remove 'uninstall' event from list of events to register

Issue mozilla#157's patch removed this event from METRICS.md, but failed to remove it from the code. The uninstall event is recorded by the Addons Manager. It is not possible to record from a WE, as the extension is uninstalled before the event can be recorded.

src/telemetry/extension.js

@@ -92,12 +92,6 @@ const EVENTS = {
       ...DEFAULT_EXTRAS,
     ],
   },
-  // User uninstalls the extension
-  uninstall: {
-    methods: ['uninstall'],
-    objects: ['uninstall'],
-    extra_keys: ['tracked_prods'],
-  },
   // User hides the toolbar button for the extension
   hide_toolbar_button: {
     methods: ['hide_toolbar_button'],