CI: Update after changes in mpi4py improving ABI support

Author: dalcinl

.github/workflows/mpi4py.yml

@@ -49,7 +49,6 @@ jobs:
     - name: Install mpi4py
       run: python -m pip install ./mpi4py
       env:
-        CPPFLAGS: "-DMPI_ABI=1"
         CFLAGS: "-O0"
 
     - name: Check mpi4py symbols
@@ -84,7 +83,6 @@ jobs:
     - name: Install mpi4py (small-count)
       run: python -m pip install ./mpi4py
       env:
-        CPPFLAGS: "-DMPI_ABI=1"
         CFLAGS: "-O0"
 
     - name: Check mpi4py symbols (small-count)