Bug 947217 - [Messages][Drafts] Consider removing the 'Drafts.request()' call in MessageManager's init

Retested this with the latest master a17a4710def9d760d9e5dc3762cdda775f1c59d0 and it's no longer necessary to make this call in MessageManager init()

- `message_manager.js`
    - removed call to `Drafts.request()`

- `message_manager_test.js`
    - removed relevant tests