Mojo: removing support for process-group.

services/service_manager/tests/lifecycle/package_app_a_manifest.json

 {
   "name": "service:lifecycle_unittest_package_app_a",
-  "process-group": "package",
   "display_name": "Lifecycle Unittest Package App A",
   "interface_provider_specs": {
     "service_manager:connector": {
       "provides": {
         "lifecycle_unittest:lifecycle_control": [
           "service_manager::test::mojom::LifecycleControl"
         ]
       }
     }
   }
 }