This test verifies the following line of code:
$(document).trigger($.Event('subscription_remove_done.zulip', {sub: sub}));
Before this change, the mocking mechanism used `$(document).on(...)`
to set the function that gets called on trigger, but it didn't clear
any of the other handlers.
Since all we care about is making sure that the event gets triggered,
we now just override `$(document).trigger`.