Can we remove all_classes?

[Joachim Durchholz]

It's not in the `sympy` namespace, but it is documented as a part of
`sympy.core.core` (not intentionally I suppose, it's just automatically
extracted).

I.e. the question is whether we guarantee a stable API for just the
`sympy` namespace, or for all submodules as well.

Motivation: I'm currently trying to eliminate metaclasses by removing
unneeded features that they implement, this should make it *much* easier
to clean up the metaclass mess we have. (Everything under
ManagedProperties is using BasicMeta both as a superclass and a
metaclass, which means that __init__ and __new__ are called both for
class creation and object creation - eww.)