Remove ci-tests for dune-2.6, as we only want to supprt dune 2.7 and higher from now on

.gitlab-ci.yml

@@ -28,12 +28,6 @@ before_script: &before
              }
     EOT
 
-dune:2.6 gcc:
-  image: registry.dune-project.org/docker/ci/dune:2.6-debian-10-gcc-8-17
-  script: duneci-standard-test
-  variables:
-    DUNECI_BRANCH: releases/2.6-1
-
 dune:2.7 gcc:
   image: registry.dune-project.org/docker/ci/dune:2.7-debian-10-gcc-8-17
   before_script: