Disabling ignoring unused vars for _ prefix

[gaearon]

We currently ignore unused vars with _ prefix, mainly as a workaround for deleting extra properties in ...rest destructuring. However #1705 is a proper fix for this, so I think we should remove the _ rule in the next major bump. This would fix #1387.

[gaearon]

Also, why do we have vars: 'local' rather than the default 'global' setting?

[sidoshi]

I can submit a PR.
Though I am not sure about the vars setting. Can't find a use case of having an unused global variable.

[gaearon]

If you could go through the git blame history for who added it and why, that could help. Or maybe there was no good reason at all.

[sidoshi]

Seems like there was no good reason.
It was there since custom eslint rules were added in the begining. See #27

[gaearon]

We probably took it here then. Would be nice to learn why Airbnb config specifies this.

[gaearon]

Maybe @ljharb could say!

[jamesblight]

The default setting in eslint was local at the time Airbnb set their config. Eslint later changed the default to all (global) and I'm guessing Airbnb never updated.

Edit: For context this is why it changed from local to all

[sidoshi]

@gaearon should the vars setting be set to default?