[ci] remove default yml

use the generator script from the template instead

.gitlab-ci/default.yml

-default:
-  image: $IMAGE
-
-stages:
-  - build
-  - test
-
-workflow:
-  rules:
-    - if: $CI_PIPELINE_SOURCE=="parent_pipeline"
-
-build dumux:
-  stage: build
-  script:
-    - dunecontrol --opts=$DUNE_OPTS_FILE --current all
-    - dunecontrol --opts=$DUNE_OPTS_FILE --current make -k -j8 build_tests
-  artifacts:
-    paths:
-      - build-cmake
-    expire_in: 3 hours
-
-test dumux:
-  stage: test
-  script:
-    - cd build-cmake
-    - dune-ctest -j8 --no-tests=error
-  needs:
-    - job: build dumux
-      artifacts: true
-  artifacts:
-    reports:
-      junit: junit/dumux-cmake.xml