[Review] Remove Multi

### Problem/Solution

We no longer support multiple requests, so any reference to `multi` should be removed from the gem.

### Notes

Relates to issue [#15](https://github.com/BBC-News/alephant-broker/issues/15) raised by @revett.

This will require a new **major release**.

**Important**: please keep this PR and branch as we may need to re-implement this functionality in the future.

I'm tethering from my phone whilst on the train to Devon, so no GIF - sorry :steam_locomotive: