GH-127381: pathlib ABCs: remove uncommon `PurePathBase` methods (#127853)

Remove `PurePathBase.relative_to()` and `is_relative_to()` because they
don't account for *other* being an entirely different kind of path, and
they can't use `__eq__()` because it's not on the `PurePathBase` interface.

Remove `PurePathBase.drive`, `root`, `is_absolute()` and `as_posix()`.
These are all too specific to local filesystems.