Don't try to decode responses from HTTP DELETE calls to the API.

Successful DELETE on an API resource will return HTTP 204 and an empty payload.
In the case where this call succeeds, we shouldn't try to JSON decode the
result because it won't work.

Testing done:
Deleted a repository using the RBTools API and didn't see the traceback
anymore.

Fixes bug 3450.

Reviewed at https://reviews.reviewboard.org/r/6097/