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>

.gitlab-ci.yml

@@ -373,15 +373,6 @@ build-user-plugins:
     MAKE_CHECK_ARGS: check-tcg
   timeout: 1h 30m
 
-build-user-centos7:
-  extends: .native_build_job_template
-  needs:
-    job: amd64-centos7-container
-  variables:
-    IMAGE: centos7
-    CONFIGURE_ARGS: --disable-system --disable-tools --disable-docs
-    MAKE_CHECK_ARGS: check-tcg
-
 build-some-softmmu-plugins:
   extends: .native_build_job_template
   needs: