Remove workflow run on master

Author: Kobzol

.github/workflows/ci.yml

@@ -11,10 +11,6 @@ name: CI
 on:
   push:
     branches:
-      # CI on master only serves for caching citool builds for the `calculate_matrix` job.
-      # In order to use GHA cache on PR CI (and auto/try) jobs, we need to write to it
-      # from the default branch.
-      - master
       - auto
       - try
       - try-perf