Move MPICH to pkgs/mpich and set as default

pkgs/overlay.nix

@@ -1,35 +1,5 @@
 final: prev:
 {
-  # Set MPICH as default
-  mpi = final.mpich;
-
-  # Configure the network for MPICH
-  mpich = with final; let
-    # pmix comes with the libraries in .out and headers in .dev
-    pmixAll = symlinkJoin {
-      name = "pmix-all";
-      paths = [ pmix.dev pmix.out ];
-    };
-  in prev.mpich.overrideAttrs (old: {
-    buildInput = old.buildInputs ++ [
-      libfabric
-      pmixAll
-    ];
-    configureFlags = [
-      "--enable-shared"
-      "--enable-sharedlib"
-      "--with-pm=no"
-      "--with-device=ch4:ofi"
-      "--with-pmi=pmix"
-      "--with-pmix=${pmixAll}"
-      "--with-libfabric=${libfabric}"
-      "--enable-g=log"
-    ] ++ lib.optionals (lib.versionAtLeast gfortran.version "10") [
-      "FFLAGS=-fallow-argument-mismatch" # https://github.com/pmodels/mpich/issues/4300
-      "FCFLAGS=-fallow-argument-mismatch"
-    ];
-  });
-
   slurm = prev.slurm.overrideAttrs (old: {
     patches = (old.patches or []) ++ [
       # See https://bugs.schedmd.com/show_bug.cgi?id=19324