Remove useMutableSource (#27011)

## Summary

This PR cleans up `useMutableSource`. This has been blocked by a
remaining dependency internally at Meta, but that has now been deleted.

<!--
Explain the **motivation** for making this change. What existing problem
does the pull request solve?
-->

## How did you test this change?

```
yarn flow
yarn lint
yarn test --prod
```

<!--
Demonstrate the code is solid. Example: The exact commands you ran and
their output, screenshots / videos if the pull request changes the user
interface.
How exactly did you verify that your PR solves the issue you wanted to
solve?
  If you leave this empty, your PR will very likely be closed.
-->