useEffect returning promises.

[abhi3315]

Is your feature request related to a problem? Please describe.
I was working on the Newspack plugin and come to know that there are a few useEffect hooks which are returning promises.
It's best practice to either return nothing or a clean-up function.
There are a few warnings too in the console.

Describe the solution you'd like
This is a simple improvement. I would like to contribute to this.

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
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[miguelpeixe]

Thank you for reporting, @abhi3315

I believe we have tackled all the instances with incorrect useEffect usage. Please, feel free to reopen if another one is identified.