abi: Update after changes in MPI ABI support

Author: dalcinl

.github/workflows/abi.yml

@@ -81,7 +81,6 @@ jobs:
     - name: Build mpi4py wheel
       run:  python -m pip wheel ./mpi4py
       env:
-        CPPFLAGS: "-DMPI_ABI=1"
         CFLAGS: "-O0"
 
     - name: Upload mpi4py wheel