gh-69686: Remove untrue part of `__import__` replacement docs (#143261)

Remove untrue part of `__import__` replacement docs

The original statement effectively says that replacing `__import__` at global scope affects import statements, and not only that, but only import statements within the rest of the executing module. None of that has been true since at least Python 2.7, I think.

This was likely missed in python/cpython#69686.