ci: drop mipsel on Debian Sid

This port was discontinued and purged from the archives:

https://lists.debian.org/debian-devel-announce/2023/09/msg00000.html

Reviewed-by: Andrea Bolognani <abologna@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>

ci/manifest.yml

@@ -144,10 +144,6 @@ targets:
         containers: false
         builds: false
 
-      - arch: mipsel
-        containers: false
-        builds: false
-
       - arch: ppc64le
         containers: false
         builds: false