Mojo: removing support for process-group.

services/file/manifest.json

 {
   "name": "service:file",
-  "process-group": "browser",
   "display_name": "File Service",
   "interface_provider_specs": {
     "service_manager:connector": {
       "provides": {
         "file:filesystem": [ "file::mojom::FileSystem" ],
         "file:leveldb": [ "leveldb::mojom::LevelDBService" ]
       },
       "requires": {
         "*": [ "app" ]
       }
     }
   }
 }