Cygwin: drop disabled O_TMPFILE POSIX unlink code

Cygwin: drop disabled O_TMPFILE POSIX unlink code

The commit message of commit 07e0a9584f9a5b2668c767ede0482a5fba498731
and the expectation set therein, are wrong.

There's no POSIX semantics allowing to link a file with a link
count of 0 and making it available in the file system again.
In fact, the Linux linkat extension AT_EMPTY_PATH explicitely
disallows to link a file descriptor to a file with a link count
of 0, except for O_TMPFILE without O_EXCL.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>