Clean up (two) old branches

[Here](https://github.com/hardbyte/python-can/branches/all) are all breaches (and some useful stats like the last activity and diff to `develop`). Most are release branches, some are open PRs. And two are neither and we should either open an PR for them or remove them to keep things clear:
- [`cperkulator-develop`](https://github.com/hardbyte/python-can/tree/cperkulator-develop)
- [`remove-MultiRateCyclicSendTaskABC`](https://github.com/hardbyte/python-can/tree/remove-MultiRateCyclicSendTaskABC)

There's also #688 but it is at least clear what the intent was, albeit it is now abandoned.