Stop testing with gcc-6 / C++-14

Dune does not support this anymore post version 2.7.

.gitlab-ci.yml

@@ -13,7 +13,3 @@ 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