ci: Fix syntax errors in workflow file

.github/workflows/python-app.yml

@@ -28,7 +28,7 @@ jobs:
         python -m pip install --upgrade pip
         python -m pip install flake8 pytest
         python -m pip install .
-        if [ -f requirements.txt ]; then pip install -r requirements.txt; fi
+        pip install -r requirements.txt
     - name: Lint with flake8
       run: |
         # stop the build if there are Python syntax errors or undefined names