Drop gitlab CI build

gitlab now provides only 400 CI minutes on free tier. This is useless as
our hakyll build + deployment takes about an hour. Just drop this as we
anyways now have our own domain and hosting.

.gitlab-ci.yml

@@ -1,21 +0,0 @@
-image: haskell:8.6.3
-
-variables:
-  STACK_ROOT: $CI_PROJECT_DIR/.stack
-
-pages:
-  cache:
-    paths:
-      - _cache
-      - .stack
-      - .stack-work
-  before_script:
-    - stack --system-ghc install --only-dependencies
-    - stack --system-ghc build
-  script:
-    - stack --system-ghc exec site build
-  artifacts:
-    paths:
-      - public
-  only:
-    - master