userfaultfd: remove UFFD_CLOEXEC, UFFD_NONBLOCK, and UFFD_FLAGS_SET

UFFD_CLOEXEC, UFFD_NONBLOCK, and UFFD_FLAGS_SET have been unused since
they were added in commit 932b18e0 ("userfaultfd:
linux/userfaultfd_k.h").  Remove them and the associated BUILD_BUG_ON()
checks.

Link: https://lkml.kernel.org/r/20250619-uffd-fixes-v3-4-a7274d3bd5e4@columbia.edu


Signed-off-by: Tal Zussman <tz2294@columbia.edu>
Acked-by: David Hildenbrand <david@redhat.com>
Acked-by: Peter Xu <peterx@redhat.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Jan Kara <jack@suse.cz>
Cc: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>