Bug 28200: Remove unit test using abbreviated format

Bug 28200: Remove unit test using abbreviated format

This patch removes the unit test that uses the abbreviated format,
since it's not supported in newer versions of Net::Netmask without
a workaround.

Test plan:
0. Set up Koha dev environment on Debian 11
1. perl t/Koha/Middleware/RealIP.t
2. Tests should pass

Signed-off-by: Victor Grousset/tuxayo <victor@tuxayo.net>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Alvaro Cornejo <cornejo.alvaro@gmail.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>