.github/workflows/cla.yml: don't run on issue comments.

This was from the original example at:

     https://github.com/contributor-assistant/github-action?tab=readme-ov-file#1-add-the-following-workflow-file-to-your-repository-in-this-pathgithubworkflowsclayml

But it seems useless - for example CLA has never actually posted anything to
any issue.

.github/workflows/cla.yml

@@ -1,7 +1,5 @@
 name: "CLA Assistant"
 on:
-  issue_comment:
-    types: [created]
   pull_request_target:
     types: [opened,closed,synchronize]