pathlib ABCs: remove caching of path parser case sensitivity (#130194)

Remove the caching `_is_case_sensitive()` function. 

The cache used to speed up `PurePath.[full_]match()` and `Path.[r]glob()`,
but that's no longer the case - these methods use
`self.parser is posixpath` to determine case sensitivity.

This makes the `pathlib._abc` module a little easier to backport to Python
3.8, where `functools.cache()` is unavailable.