Documentation/ABI: move sysfs-kernel-uids to removed directory

commit 7c941438 ("sched: Remove USER_SCHED") deleted the
USER_SCHED feature. so move the ABI doc to removed directory.

Signed-off-by: Wang Long <w@laoqinren.net>
Link: https://lore.kernel.org/r/1581082930-30441-1-git-send-email-w@laoqinren.net


Signed-off-by: Jonathan Corbet <corbet@lwn.net>