fix: unsubscribe needs to be called

[peterferguson]

The unsubscribe function should either be called or removed since it is immediately followed by a reset of the observer which runs removeObserver without the hasListeners guard of unsubscribe.

Arguably it is better to only call unsubscribe and remove the reset. Can update the PR if you agree

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[kalijonn]

The unsubscribe function needs to be removed. Tanstack query creates a new observer when a component is unmounted and remounted again. In jotai, we keep the observer even when unmounted. To achieve the same behavior as creating a new observer, we are resetting the observer. This causes the mutation (in the observer) to be set to undefined. On mount, the subscription happens again leading to the observer now observing a new mutation.

I have cleaned it up as part of #91