Custom Field documentation mentions __metaclass__ incorrectly.

[Matthew Schinckel]

The page ​https://docs.djangoproject.com/en/dev/howto/custom-model-fields/#useful-methods specifically says: "Once you’ve created your Field subclass and set up the __metaclass__,", but metaclasses are not mentioned prior to this.

I think this is something that should have been changed with the phasing out of the SubFieldBase metaclass.

[Tim Graham <timograham@…>]

In 6d7784a7:

Fixed #24602 -- Removed obsolete reference to metaclass in custom model fields docs.

Thanks schinckel for the report.

[Tim Graham <timograham@…>]

In d11e87bf:

[1.8.x] Fixed #24602 -- Removed obsolete reference to metaclass in custom model fields docs.

Thanks schinckel for the report.

Backport of 6d7784a7b994eb8287ec851b22acb5170e77a90a from master