mypy: remove has_member (#8438)

[sthagen]

In particular:

• The test case mentioned in the code passes without it
• The test case changed seems to have more desirable behaviour now,
  consider:

from typing import Any

"""
class C:
    def __radd__(self, other) -> float:
        return 1.234
"""
C: Any
class D(C):
    pass

reveal_type("str" + D())