Fix jetstream removeSub logic

[matthewdevenny]

Signed-off-by: Matthew DeVenny matt@boxboat.com

When removeSub calls jsi.hbc.Stop() it is possible that the timer has already expired
https://github.com/nats-io/nats.go/blob/main/nats.go#L3919

from timer.Stop()

For a timer created with AfterFunc(d, f), if t.Stop returns false, then the timer has already expired 
and the function f has been started in its own goroutine; Stop does not wait for f to complete before 
returning. If the caller needs to know whether f is completed, it must coordinate with f explicitly.

This PR checks to see if sub.closed == true and returns

[coveralls]

Coverage decreased (-0.1%) to 85.055% when pulling 2c63528 on boxboatmatt:fix-activity-check into b337a5c on nats-io:main.

[kozlovic]

LGTM. Thank you for your contribution!