remove: timeout on removing all tasks

[oliver-sanders]

Closely related to #5231

If a user removes all tasks in a workflow e.g. cylc remove *, then there are no tasks left in the pool and the scheduler will shut down.

This is the correct behaviour, however, it can be cumbersome, e.g. if the user was intending to trigger new tasks (e.g. warm-start-like use cases) e.g:

$ cylc remove '*'
$ cylc trigger <cycle>/<start-task> [--flow=new]

To get around this, users must pause the workflow before removing tasks, then unpause it after. To make this easier, we could consider using a timeout similar to how we handle the similar use case of restarting a completed workflow.

[hjoliver]

(Agreed offline to do this).