common/gadi/packages.yaml: use system openmpi directly

* This commit requires ACCESS-NRI/spack-packages#66
  for ACCESS-OM2 to build on Gadi.

common/gadi/packages.yaml

@@ -2,7 +2,7 @@ packages:
   all:
     providers:
       # Completely ignoring higher-level configuration options is supported with the :: notation for keys ...
-      mpi:: [nci-openmpi, openmpi]
+      mpi:: [openmpi]
   perl:
     externals:
     - spec: [email protected]~cpanm+shared+threads
@@ -13,9 +13,9 @@ packages:
     - spec: [email protected]
       prefix: /apps/cmake/3.24.2
     buildable: false
-  nci-openmpi:
+  openmpi:
     externals:
-    - spec: nci-[email protected]
+    - spec: [email protected]
       prefix: /apps/openmpi/4.0.2
       modules:
       - openmpi/4.0.2