ci: remove unnecessary actions/setup-python

We're fine with the one in the image, and weren't specifying a version
to install, so that's what we had even with setup-python. Along with a
warning that we didn't specify a version.

Author: scop

.github/workflows/check.yml

@@ -11,7 +11,6 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v4
-      - uses: actions/setup-python@v4
       # No caching as we're expected to run rarely
       - name: Install dependencies and run checks
         run: |