Disable "MPI-enabled" CI tests

They no longer serve a purpose besides producing false negatives and burning up our free hours with GitHub's runner

Author: mjkramer

.github/workflows/test.yml

@@ -16,7 +16,7 @@ jobs:
         shell: bash -l {0}
     strategy:
       matrix:
-        mpi_enabled: [0, 1]
+        mpi_enabled: [0]
       fail-fast: false
 
     steps: