trigger: remove --on-resume option

[oliver-sanders]

The --on-resume option was added as a temporary bodge to ensure that a certain use case would continue to function after the "triggering a task whilst the workflow is paused causes it to run instantly" change.

This option is scheduled for demolition at 8.5.0. Ensure that inter-operability is as functional as we can get it with this change.

For context, see #6499

[wxtim]

This will be safe to remove once we've sorted out broadcasting to past tasks, such that a broadcast to a N>1 window past task isn't immediately cancelled.

2025-07-07T13:00:22+01:00 INFO - [2012/foo:waiting] => preparing
2025-07-07T13:00:23+01:00 INFO - Command "broadcast" received.
    broadcast(cycle_points=['2000'], mode=put_broadcast, namespaces=['foo'], settings=[{'script': 'echo Hello World'}])
2025-07-07T13:00:23+01:00 INFO - Broadcast set:
    + [2000/foo] script=echo Hello World
2025-07-07T13:00:23+01:00 INFO - Broadcast cancelled:
    - [2000/foo] script=echo Hello World

[oliver-sanders]

So not in time for 8.5.0 :(