Correct deletion of additional speaker

Fixes this error from earlier today:

```
  File "/srv/staging-pycon.python.org/releases/5c32ea44f43c367f98f9000f91b0089a496824fc/symposion/proposals/views.py", line 307, in proposal_leave
    proposal.additional_speakers.remove(speaker)

AttributeError: 'ManyRelatedManager' object has no attribute 'remove'
```