Stop running gh-pages on pull requests

It is not needed and the build part is taken care of by the netlify
workflow anyways.

Author: mbaldessari

.github/workflows/gh-pages.yml

@@ -4,7 +4,6 @@ on:
   push:
     branches:
       - main  # Set a branch to deploy
-  pull_request:
 
 jobs:
   deploy: