test(snap): remove centos 10 from test loop

CentOS 10 cloud image has a truncated checksum file causing each download
to fail validation. Let's remove it from spread.yaml while keeping the
.image-garden.mk entry for another day.

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>

Author: zyga

spread.yaml

@@ -29,9 +29,6 @@ backends:
             - fedora-cloud-44:
                   username: fedora
                   password: fedora
-            - centos-cloud-10:
-                  username: centos
-                  password: centos
 
 exclude:
     - ".cache/*"