Remove the warning for setState on unmounted components

[chriscerie]

Consistent with upstream React, we should remove the error message when you setState on an unmounted component.

In summary, while it can warn on potential memory leaks, false positives is common and can push people to undesirable workarounds:

function TestComponent:didMount()
    testPromise():andThen(function(newState)
        -- Component might unmount before our promise resolves
        self:setState(newState)
    end)
end

For the full reasoning behind removing the warning, see Dan Abramov's PR: facebook/react#22114

Checklist before submitting:

• Added entry to CHANGELOG.md
• Added/updated relevant tests
• Added/updated documentation

[coveralls]

Coverage decreased (-0.004%) to 94.783% when pulling c6388dd on chriscerie:remove-setstate-warning into 0f26436 on Roblox:master.

[github-actions]

CLA Signature Action: All authors have signed the CLA. You may need to manually re-run the blocking PR check if it doesn't pass in a few minutes.

[matthargett]

One small nit, overall looks great. Thanks for bringing this upstream React improvement over!