Remove handler assertion in removeListener

[akiroz]

Qlobber supports removing all listeners from a topic by not passing a specific listener.

Removed handler assertion in removeListener to support this use case.

[mcollina]

Can you please add a unit test?

[mcollina]

Unfortunately changing the API of this module has a pretty wide reach: all the implementers of the same interface must add its support. It's better if you add a new removeAllListeners method, so it's easier to detect if it's there or not.

[akiroz]

@mcollina Thanks for the feedback, I have added removeAllListeners instead with tests

[mcollina]

lgtm