KAFKA-19683: Clean up TaskManagerTest (#20994)

Final cleanup of `TaskManagerTest.java`
- Renamed `setUpTaskManagerWithStateUpdater` to `setUpTaskManager`
- Removed `WithStateUpdaterEnabled` from test names
- Removed the method `setUpTaskManagerWithoutStateUpdater`
- Rewrote the newly added test
`shouldCloseTasksIfStateUpdaterTimesOutOnRemove` to use new mocking
patterns
- Completely removed the utility class `StateMachineTask` and also
`handleAssignment` method

Reviewers: Lucas Brutschy <lbrutschy@confluent.io>

Author: shashankhs11