Remove pull request processing from prow action (#3445)

The prow action does not support running commands except for /lgtm in
PRs.

Signed-off-by: Nolan Brubaker <brubakern@vmware.com>

.github/workflows/prow-action.yml

@@ -4,10 +4,6 @@ name: "Prow github actions"
 on:
   issue_comment:
     types: [created]
-  pull_request:
-    types:
-      - opened
-      - ready_for_review
 
 jobs:
   execute: