self.master.data.updates.completeBuildRequests was called twice

Each time a buildrequest is completed, in twistd.log I have: 
```
tried to complete 1 buildrequests, but only completed 0
```
This log line appears, because self.master.data.updates.completeBuildRequests is called twice.
hum this was weird implementation. I wonder why it was written like this in the first place.

@djmitche ideas?