docs: Remove remainders of reiserfs

docs: Remove remainders of reiserfs

Reiserfs has been removed in 6.13, there are still some mentions in the
documentation about it and the tools. Remove those that don't seem
relevant anymore but keep references to reiserfs' r5 hash used by some
code.

There's one change in a script scripts/selinux/install_policy.sh but it
does not seem to be relevant either.

Signed-off-by: David Sterba <dsterba@suse.com>
Acked-by: Paul Moore <paul@paul-moore.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20250813100053.1291961-1-dsterba@suse.com