createValidator = (isRequired) =>
      isRequired ? (value) => (value ? undefined : "Required") : undefined
Allow me to explain this. If we allow field level validation functions
to be swapped, it means that we'd have to run _ALL_ the validation
every time a new field was removed regardless of whether or
not it was using field-level validation. To avoid this overhead, we must
accept some inconsistency when swapping of field-level validation functions.
In this test, swapping from no validation to "required" validation
does work because the field-level validation function is called on mount,
but the error does not clear when we switch back to no validation function
because in order for Final Form to determine if the 'Required' error came
from the newly unmounted Field, it would need to run validation on the entire
form.