Revert of services/shell: Turn on sandboxing for mojo apps.

components/font_service/manifest.json

 {
   "manifest_version": 1,
   "name": "mojo:font_service",
   "display_name": "Font Service",
   "capabilities": {
     "provided": {
       "app": [
         "font_service::mojom::FontService"
       ]
     },
     "required": {
-      "*": { "classes": [ "app" ] },
-      "mojo:shell": { "classes": ["shell:unsandboxed"] }
+      "*": { "classes": [ "app" ] }
     }
   }
 }