Fix pip update error in CI

Fix `error: externally-managed-environment` error when running the "Setup
Python" step. The already-installed pip version in CI is new enough that this
step is no longer needed.

.github/workflows/ci.yml

@@ -42,10 +42,6 @@ jobs:
     - name: Docs
       run: |
         make docs
-    - name: Setup Python
-      run: |
-        pip install --upgrade pip
-      if: ${{ matrix.os == 'ubuntu-latest' }}
     - name: Python Lint
       run: |
         cd rten-convert