ci(workflows): switch deployment branch

Remove old deployment branch with project name prefix,
only deploy from "deploy" going forward.

.github/workflows/deploy-apis-instance.yml

@@ -3,7 +3,8 @@ name: Deploy Frischmuth app
 on:
   push:
     # run deployment only on pushes to specific branches
-    branches: ["deploy", "frischmuth/deploy"]
+    branches:
+      - deploy
   # for triggering deployments manually via button in GitHub interface
   workflow_dispatch: