Remove [`StubTreq`](https://github.com/rackerlabs/otter/blob/b4b858a186660e095bb6290bc48e659172cf41bd/otter/test/utils.py#L412) and [`StubTreq2`](https://github.com/rackerlabs/otter/blob/b4b858a186660e095bb6290bc48e659172cf41bd/otter/test/utils.py#L485) and use one [provided in treq](https://github.com/twisted/treq/blob/fcf5deb976c955ca6ef6484f414d25839932940e/src/treq/testing.py#L184) package itself.
