Delete old meet hack from checkmember.py (#18662)
The hack to use `meet_types(original_type, itype)` to select a correct
element from a union appeared before we added proper handling of unions
in various places related to `checkmember.py`. This is error prone,
since `meet_types()` is one of least precise type ops (for good and bad
reasons), and results in obscure bugs, see e.g.
https://github.com/python/mypy/issues/15600
This hack should not be needed anymore, now we have three-level
information available everywhere we needed it:
* `original_type` - as the name says, a type from which everything
started. This is used for error messages and for plugin hooks.
* `self_type` - a specific element of the union is the original type is
a union. The name is because this is what will be ultimately used by
`bind_self()`
* `itype` the actual instance type where we look up the attribute (this
will be e.g. a fallback if the `self_type` is not an instance)
