CI: Disable C++14 runner

The image of debian 9 with C++14 compiler is not updated any more
and incompatible with the new allowed usage of C++17 features.

.gitlab-ci.yml

@@ -14,7 +14,3 @@ dune:git  gcc-9  C++20:
 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