Revert "SDL pipelines should run even if no code changes (microsoft#236)" (microsoft#237)

Author: walbourn

build/DirectXMath-SDL.yml

@@ -8,6 +8,9 @@
 schedules:
 - cron: "0 3 * * 0,3,5"
   displayName: 'Three times a week'
+  branches:
+    include:
+    - main
   always: true
 
 # GitHub Actions handles CodeQL and PREFAST for CI/PR