Revert "PySide6 python-package workflow: offscreen"

This reverts commit 56e4e54.

Author: PierreRaybaut

.github/workflows/python-package-PySide6.yml

@@ -46,5 +46,4 @@ jobs:
         flake8 . --count --exit-zero --max-complexity=10 --max-line-length=127 --statistics
     - name: Test with pytest
       run: |
-        echo "QT_QPA_PLATFORM=offscreen" >> "$GITHUB_ENV"
         pytest