chore: cleanup unused ci&cd notifications

when we used Travis, we had a workflow/job/command/egg to handle cases where PRs are open from staging to the master branch.
this flow has been irrelevant since our migration to CircleCI.
removing its unused references and the script it used to invoke.

Author: Shesekino

.circleci/config.yml

@@ -214,16 +214,6 @@ jobs:
           command: ./scripts/slack-notify-failure.sh "deploy-dev"
           when: on_fail
 
-######################## PR TO MASTER ########################
-  prepublish:
-    <<: *default_container_config
-    steps:
-      - checkout
-      - run:
-          name: PREPUBLISH
-          command: |
-            ./scripts/slack-notify-pr.sh
-
 ######################## MERGE TO MASTER ########################
   publish:
     <<: *default_container_config