gitlab: Drop linux user build job for CentOS 7

gitlab: Drop linux user build job for CentOS 7

It has been over two years since RHEL-8 was released, and thus per the
platform build policy, we no longer need to support RHEL-7 as a build
target.

The build-user-centos7 job was to detect a failure specific to CentOS
7 and there are already other linux user jobs for other platforms.
Thus we can drop this job rather than move it to CentOS 8.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Willian Rampazzo <willianr@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20210514120415.1368922-2-berrange@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>