hydra: cuirass: Remove broken kernel-updates-images jobset.

This jobset doesn't help test the new kernel versions because the
Cuirass system on berlin.gnu.org doesn't reliably build images, failing
with "missing derivation" errors like this:

------
cannot build missing derivation ?/gnu/store/hs6kp1lqgymhyp3jndc0dsp0pn4psgv0-gui-installed-desktop-os-encrypted.drv?
------

This is bug #54447:

https://issues.guix.gnu.org/54447

* hydra/modules/sysadmin/services.scm (cuirass-specs): Remove
'kernel-updates-images'.

hydra/modules/sysadmin/services.scm

@@ -408,16 +408,6 @@ collection instead."
                 (period 7200)
                 (priority 2)
                 (systems '#$systems))
-               (specification
-                (name "kernel-updates-images")
-                (build 'images)
-                (channels
-                 (list (channel
-                        (inherit %default-guix-channel)
-                        (branch "kernel-updates"))))
-                (period 7200)
-                (priority 2)
-                (systems '#$systems))
                (specification
                 (name "kernel-updates-system-tests")
                 (build 'system-tests)