Merge branch 'ci/disable-debian-9' into 'master'

CI: Disable Debian 9 runner

See merge request !61

.gitlab-ci.yml

@@ -12,7 +12,3 @@ dune:git  clang  C++17:
 dune:git  gcc-8  C++17:
   image: registry.dune-project.org/docker/ci/dune:git-debian-10-gcc-8-17
   script: duneci-standard-test
-
-dune:git  gcc-6  C++14:
-  image: registry.dune-project.org/docker/ci/dune:git-debian-9-gcc-6-14
-  script: duneci-standard-test