Revert 09b3695 (re. browser.alarms)

Relates issue:
- uBlockOrigin/uBlock-issues#2591

THe issue was simply that I had forgotten to add the `alarms`
permission to the manifest.

platform/common/vapi-background.js

@@ -95,45 +95,6 @@ vAPI.storage = webext.storage.local;
 /******************************************************************************/
 /******************************************************************************/
 
-// https://github.com/uBlockOrigin/uBlock-issues/issues/2591
-//   Report of alarms API not being supported on Thunderbird
-if ( browser.alarms === undefined ) {
-    browser.alarms = {
-        alarmsMap: new Map(),
-        listenerSet: new Set(),
-        create(name, delayInfo) {
-            let alarm = this.alarmsMap.get(name);
-            if ( alarm !== undefined ) {
-                alarm.off();
-            } else {
-                alarm = vAPI.defer.create(( ) => {
-                    this.alarmsMap.delete(name);
-                    for ( const listener of this.listenerSet ) {
-                        listener({ name });
-                    }
-                });
-            }
-            this.alarmsMap.set(name, alarm);
-            alarm.on({ min: delayInfo.delayInMinutes });
-        },
-        clear(name) {
-            const alarm = this.alarmsMap.get(name);
-            if ( alarm === undefined ) { return; }
-            alarm.off();
-            this.alarmsMap.delete(name);
-        },
-        get: function(name, callback) {
-            callback(this.alarmsMap.has(name) && { name } || undefined);
-        },
-        onAlarm: {
-            addListener(callback) {
-                browser.alarms.listenerSet.add(callback);
-            },
-        },
-    };
-}
-
-
 vAPI.alarms = {
     create(callback) {
         this.uniqueIdGenerator += 1;

platform/thunderbird/manifest.json

@@ -66,6 +66,7 @@
     "open_in_tab": true
   },
   "permissions": [
+    "alarms",
     "privacy",
     "storage",
     "tabs",