refactor: remove redundant SigInt handler

[kentbull]

Since the Doist loop listens for the KeyboardInterrupt exception then it effectively handles the Ctrl-C SigInt interrupt already which means the shutdown handler only has to handle the SigTerm signal.

[SmithSamuelM]

As I posted on the other related repo, The Doer sigterm handler should either raise KeyboardInterrupt or call sys.exit() to trigger doist.exist instead of calling doist.exit() directly.

[kentbull]

Thanks for pointing that you. Yes, I remember now, you did say that. I'll post a PR later this week with my draft at having an agency properly close itself. Having the GracefulShutdownDoer raise a KeyboardInterrupt is the easy part. The hard part is in having each Agent and then the Agency shut themselves down in the proper order.

[SmithSamuelM]

The ordering is already taken care of by the Doist. The order of the enter calls is reversed for the exit calls. So if you enter doers in the order of dependency where if C is dependent on B and B is dependent on A then you want to enter them in the order A - B - C. Then when doist.exit gets called it will exit its doers in the reverse order. C - B - A. This avoids race conditions where C depends on a resource in order to exit cleanly but that resource has already exited.