User metadata is cached in twice

[mdawaffe]

Since switching the usermeta to the new meta API, user metadata is cached in two different places.

Once in the user object in the users cache group via _fill_user and friends.

Once in the user_meta cache group via the meta API.

[scribu]

I say drop _fill_user().

[ryan]

I'd like to drop meta caching in the user object, but it is pretty widely used.

[ryan]

We have plans for this in 3.2.

[duck_]

Fixed in #15458.