patch(_sync_docs.yaml): Remove on push from usage docs (#181)

Sync from GitHub -> Discourse is currently disabled; no need to sync on
`push` for now


canonical/postgresql-k8s-operator#495 (comment)

.github/workflows/_sync_docs.md

@@ -15,9 +15,6 @@ on:
   workflow_dispatch:
   schedule:
     - cron: # Refer to Run schedule below
-  push:
-    branches:
-      - main
 
 jobs:
   sync-docs:
@@ -74,4 +71,4 @@ jobs:
 |:------------------:|:--------:|:-------------:|
 | data-integrator    | 04:10 AM | `10 04 * * *` |
 | s3-integrator      | 04:20 AM | `20 04 * * *` |
-| data-platform-libs | 04:30 AM | `30 04 * * *` |
+| data-platform-libs | 04:30 AM | `30 04 * * *` |