Change CodeQL analysis on cron only (#7229)

now that they mark unrelated changes as failures on PR...

.github/workflows/codeql-analysis.yml

@@ -1,10 +1,6 @@
 name: "CodeQL"
 
 on:
-  push:
-    branches: [ main ]
-  pull_request:
-    branches: [ main ]
   schedule:
     - cron: '0 8 * * 3'