fix(auth): cannot delete property 'uid' of #<UserInfo>

[Youmoo]

Hi. This error (Cannot delete property 'uid' of #<UserInfo>) occurs when we do something like below:

await auth.updateUser(uid, {
  providerToLink: {
    providerId: 'google.com',
    uid: 'google_uid',
  },
});
const user = await auth.getUser(uid);
const provider = user.providerData.find(v => v.providerId === 'google.com');
// previously this will throw an error: Cannot delete property 'uid' of #<UserInfo>
const updated = await auth.updateUser(uid, {
  providerToLink: provider,
});

The use case is I want to unlink a given provider from user A and then link that same provider to user B.