Remove GitLab CI configuration

As noted by @renchap, "We do not rely on Gitlab CI anymore, this
can be dropped."

https://github.com/mastodon/documentation/issues/1357#issuecomment-1851642299

.gitlab-ci.yml

@@ -1,13 +0,0 @@
-image: monachus/hugo:latest
-
-variables:
-  GIT_SUBMODULE_STRATEGY: recursive
-
-pages:
-  script:
-    - hugo
-  artifacts:
-    paths:
-      - public
-  only:
-    - master