Fix actually remove openURL event listener

[joeybaker]

Previously, we were creating an anonymous function as the event listener handler. This means we can't un-listen because we don't have a reference to the handler.

[vonovak]

@matthamil can we get this merged?

ad additional description: the important part that got removed is:

Linking.addEventListener('url', ({ url }: { url: string }) => {
    this._handleOpenURL(url);
});

^ the function passed to addEventListener is anonymous.
however, Linking.removeEventListener('url', this._handleOpenURL); is called here which of course doesn't work, because this._handleOpenURL never got registered in the first place!

[vonovak]

@spencercarli in an effort to decrease the number of open PRs, I'd like to draw attention to the fact that this PR can be merged

[vonovak]

@charpeni I remeber reading somewhere that you have push access to this repo. Can you please take a look at this? thanks

[spencercarli]

Thanks @joeybaker!