Retiring autoload hook for enums in python

Enums can now be natively autoloaded by ROOT, so I am cleaning up my own old autoloading implementation based on the ROOT6_NamespaceAutoloadHook object presence.
Removed: implementation in the python import hook and all occurrences of ROOT6_NamespaceAutoloadHook in the headers and selection files.

NOTE: ROOT autoloading is based on actual enum type names, so the first use of the enum must mention that name. After the enum is imported, it will be also visible directly in the enclosing namespace, like in C++. I changed the code in the helpers importing enums to take that into account. That way, no user code needs to be modified (assuming they are using the helper).