Revert "ci: try adding slc6 image"

This reverts commit 64a05ae1.

.gitlab-ci.yml

@@ -2,22 +2,6 @@ stages:
 - build
 - publish
 
-slc6:
-  stage: build
-  image: gitlab-registry.cern.ch/linuxsupport/slc6-base
-  script:
-  - yum install -y rpm-build git
-  - git fetch --tags
-  - git submodule update --recursive --init
-  - git tag
-  - git describe --dirty
-  - ./make-rpm.sh
-  - mkdir ${CI_JOB_NAME}
-  - cp -r /root/rpmbuild/RPMS build/SRPMS ${CI_JOB_NAME}
-  artifacts:
-    paths:
-    - "$CI_JOB_NAME"
-
 cc7:
   stage: build
   image: gitlab-registry.cern.ch/linuxsupport/cc7-base