Auth server doesn't delete headless auth request when tsh --headless command is canceled

[ravicious]

1. Log in to a cluster in Connect.
2. Outside of Connect, execute a tsh command with --headless.
3. Wait for the headless auth modal to show up in Connect.
4. Cancel the tsh command.

At this point, the modal should disappear. This used to work in v16.0.0, but it didn't work during the v17 test plan. Initially I assumed it's a problem with grpc-js, but it also doesn't work in Connect/tsh v16 when talking to a v17 cluster. I think this points to the issue being on the server side where OpDelete is not sent to clients.

teleport/lib/teleterm/daemon/daemon_headless.go

Lines 201 to 202 in 6ac6025

	case types.OpDelete:
	cancelPendingRequest(event.Resource.GetName())