GH-127381: pathlib ABCs: remove `WritablePath.mkdir()` arguments (#130611)

Remove the *mode*, *parents* and *exist_ok* arguments from
`WritablePath.mkdir()`. These arguments imply support for POSIX permissions
and checking for preexistence of the path or its parents, but subclasses of
`WritablePath` may not have these capabilities.

The public `Path.mkdir()` method retains these arguments.