[libc++] Fix double file closing in `std::filesystem::remove_all()`.

According to Linux documentation (see e.g. https://linux.die.net/man/3/closedir):

> A successful call to `closedir()` also closes the underlying file
> descriptor associated with `dirp`.

Thus, calling `close()` after a successful call to `closedir()` is at
best redundant. Worse, should a different thread open a file in-between
the calls to `closedir()` and `close()` and get the same file descriptor,
the call to `close()` might actually close a different file than was
intended.

rdar://89251874

Differential Revision: https://reviews.llvm.org/D120453