Remove current implementation of cancelling ongoing Actions

Currently, when an ongoing ActionPlan is cancelled by an user the behavior is:

- Failed or Successful actions stay in the same status.
- Pending actions are moved to Cancelled and are not executed.
- Ongoing actions. For some actions (live_migrations mainly), the applier provides a mechanism which tries to cancel the migration and abort it.

The current mechanism to cancel ongoing actions is based on a polling mechanism which checks the state in the database every second for every action and kills a thread where the action is being executed. This mechanism is highly inefficient in terms of performance, error prone, and managing properly the cancellation would require better support from actions.

As discussed in PTG fro 2026.1, until we have a proper mechanism cancellation mechanism the proposal is to remove current implementation.

The behavior on Cancelling action-plans for actions will be to wait for the running task to complete and cancel all pending task

The current behavior of the ActionPlan process is not documented in detail in Watcher doc. It will be added to the documentation as part of this change.