Revert "Reinstate pull_request workflow trigger"

This reverts commit ed4f1ed.

Author: sd109

.github/workflows/test-pr.yml

@@ -11,7 +11,6 @@ on:
       - reopened
     branches:
       - master
-  pull_request:
 
 # Use the head ref for workflow concurrency, with cancellation
 # This should mean that any previous workflows for a PR get cancelled when a new commit is pushed