Revert ".gitlab-ci.yml: initial commit"

This reverts commit 95177362edc9c814a102c8a2236695c632892232 to
disable GitLab CI temporarily.

.gitlab-ci.yml

@@ -1,20 +0,0 @@
-
-.build:
-  stage: build
-  image: fedora
-  before_script:
-     - dnf -y install gcc-c++ cmake make
-  script:
-    - cmake -B builddir -DEIGEN_TEST_CXX11=${EIGEN_TEST_CXX11}
-    - cmake --build builddir --target buildtests
-    - cd builddir && ctest --output-on-failure
-
-CXX11 ON:
-  variables:
-    EIGEN_TEST_CXX11: "ON"
-  extends: .build
-  
-CXX11 OFF:
-  variables:
-    EIGEN_TEST_CXX11: "OFF"
-  extends: .build