Remove use of mpi GHA step

Author: jsiirola

.github/workflows/test_branches.yml

@@ -226,12 +226,6 @@ jobs:
         echo "SETUPTOOLS_USE_DISTUTILS=local" >> $GITHUB_ENV
         choco install pkgconfiglite
 
-    - name: Install system MPI
-      if: ${{ matrix.mpi != 0 && matrix.category != 'notests' }}
-      uses: mpi4py/setup-mpi@v1
-      with:
-        mpi: openmpi
-
     - name: Set up Python ${{ matrix.python }}
       if: matrix.PYENV == 'pip'
       uses: actions/setup-python@v5

.github/workflows/test_pr_and_main.yml

@@ -278,12 +278,6 @@ jobs:
         echo "SETUPTOOLS_USE_DISTUTILS=local" >> $GITHUB_ENV
         choco install pkgconfiglite
 
-    - name: Install system MPI
-      if: ${{ matrix.mpi != 0 && matrix.category != 'notests' }}
-      uses: mpi4py/setup-mpi@v1
-      with:
-        mpi: openmpi
-
     - name: Set up Python ${{ matrix.python }}
       if: matrix.PYENV == 'pip'
       uses: actions/setup-python@v5