Remove untested triggers from the "Manage PRs" workflow

I had intended to test these out but didn't get around to it. The workflow_dispatch might be convenient, but it's not at
all essential since the same thing can be accomplished by commenting on the PR.

Author: per1234

.github/workflows/manage-prs.yml

@@ -13,10 +13,6 @@ on:
     types:
       - created
       - edited
-  # See: https://docs.github.com/en/free-pro-team@latest/actions/reference/events-that-trigger-workflows#workflow_dispatch
-  workflow_dispatch:
-  # See: https://docs.github.com/en/free-pro-team@latest/actions/reference/events-that-trigger-workflows#repository_dispatch
-  repository_dispatch:
 
 jobs:
   enabled: