Remove merge:master job entirely

Apparently disabling it by checking for a variable doesn't actually
disable it.

[ci skip]

.gitlab-ci.yml

@@ -1053,27 +1053,3 @@ schedule:review-cleanup:
     - gem install gitlab --no-document
   script:
     - ruby -rrubygems scripts/review_apps/automated_cleanup.rb
-
-merge:master:
-  image: registry.gitlab.com/gitlab-org/merge-train
-  stage: merge
-  # The global before_script/after_script blocks break this job, or aren't
-  # necessary. These two lines result in them being ignored.
-  before_script: []
-  after_script: []
-  only:
-    refs:
-      - master
-      - schedules
-    variables:
-      - $CI_PROJECT_PATH == "gitlab-org/gitlab-ce"
-      - $MERGE_TRAIN_SSH_PUBLIC_KEY
-      - $MERGE_TRAIN_SSH_PRIVATE_KEY
-      - $MERGE_TRAIN_API_TOKEN
-      - $MERGE_FORCE_ENABLE
-  script:
-    - scripts/merge-train
-  cache:
-    paths:
-      - gitlab-ee
-    key: "merge:master"