gha: remove outdated borg2 branch

.github/workflows/ci.yml

@@ -16,7 +16,7 @@ on:
     - 'requirements.d/*'
     - '!docs/**'
   pull_request:
-    branches: [ master, borg2 ]
+    branches: [ master ]
     paths:
     - '**.py'
     - '**.pyx'