Remove manifest.webapp

I think this was only used for the now defunct FireFox OS app store.
The standard manifest.json should suffice.

manifest.webapp

@@ -1,36 +0,0 @@
-{
-  "app": {
-    "background": {
-      "scripts": ["js/start.js"]
-    }
-  },
-  "author": "William Brawner",
-  "chrome": { "navigation": false },
-  "default_locale": "en",
-  "description": "An interval timer that allows you to customize the intervals",
-  "display": "standalone",
-  "developer": {
-    "name": "William Brawner",
-    "url": "https://wbrawner.com"
-  },
-  "icons": [
-    {
-      "src": "/img/android-chrome-192x192.png",
-      "sizes": "192x192",
-      "type": "image\/png"
-    },
-    {
-      "src": "/img/android-chrome-512x512.png",
-      "sizes": "512x512",
-      "type": "image\/png"
-    }
-  ],
-  "launch_path": "/index.html",
-  "manifest_version": 2,
-  "name": "Interval Timer",
-  "short_name": "Interval Timer",
-  "background_color": "#FFFFFF",
-  "offline_enabled": true,
-  "theme_color": "#fff9c4",
-  "version": "2.0"
-}