Remove "stop" signal along with Stoppable interfaces

During the ML discussion of FLIP-34 we realised that it would be beneficial for this new feature to replace the existing "stop" functionality. The current "stop" functionality cannot be used because no real-world sources support the functionality. Therefore, I think it is save to remove because it should not break existing workflows.

The issue proposes completely removing the old stop feature, introduced via FLINK-2111, as preparation for FLIP-34.

We have to be careful when doing this because it touches quite a few things. Basically, we have to do a manual revert of this commit: https://github.com/apache/flink/commit/bdd4024e20fdfb0accb6121a68780ce3a0c218c0