remove base from create pull request in themes-weekly-pull.yml

Co-authored-by: per1234 <accounts@perglass.com>

Author: francescospissu

.github/workflows/themes-weekly-pull.yml

@@ -47,4 +47,3 @@ jobs:
           title: Update themes
           branch: themes/themes-update
           author: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>
-          base: ${{ github.head_ref }}