docs: Remove outdated highuid.rst documentation

The highuid.rst document describes a transition that is outdated and no
longer relevant. Additionally, it references filesystems (ncpfs and smbfs),
which have been removed or replaced.

Suggested-by: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Kang Taeho <kangtaeho2456@gmail.com>
Link: https://lore.kernel.org/r/20250313145650.278346-1-kangtaeho2456@gmail.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Author: kth333

Documentation/admin-guide/highuid.rst

@@ -187,7 +187,6 @@ A few hard-to-categorize and generally obsolete documents.
 .. toctree::
    :maxdepth: 1
 
-   highuid
    ldm
    unicode