Remove useUser's initialData override

[jhuleatt]

Description

useUser currently overrides initialData if auth.currentUser is not undefined:

reactfire/src/auth.tsx

Lines 29 to 33 in 414201e

	// only set/override initialData if auth has finished loading
	if (auth.currentUser !== undefined) {
	_options.initialData = auth.currentUser;
	_options.startWithValue = auth.currentUser;
	}

The idea was that we might be able to get useUser to load with the correct auth state faster than if we had waited for the underlying useIdTokenResult to finish. This was based on the assumption that auth.currentUser is undefined while the Auth SDK is booting up, null only when the Auth SDK is booted up and a user isn't signed in, and truthy otherwise.

However, this is undocumented behavior, so we shouldn't rely on it.