Don't re-enable split mesh partitioning too early

This fixes libMesh/libmesh#3759 regressions in
the GridPartitioner mesh splitting tests for me.  When we recognize that
an unpartitioned mesh is unprepared (which the libMesh PR now does), and
that prompts MOOSE to call prepare_for_use(), we'd better still have
partitioning disabled unless we want to see some partitioning.

Author: roystgnr

framework/src/actions/SplitMeshAction.C

@@ -31,6 +31,9 @@ void
 SplitMeshAction::act()
 {
   auto mesh = _app.actionWarehouse().mesh();
+
+  mesh->getMesh().skip_partitioning(false);
+
   const std::string split_file_arg = _app.parameters().isParamSetByUser("split_file")
                                          ? _app.parameters().get<std::string>("split_file")
                                          : "";

framework/src/mesh/MooseMesh.C

@@ -2960,10 +2960,6 @@ MooseMesh::init()
       getMesh().skip_partitioning(true);
     buildMesh();
 
-    // Re-enable partitioning so the splitter can partition!
-    if (_app.isSplitMesh())
-      getMesh().skip_partitioning(false);
-
     if (getParam<bool>("build_all_side_lowerd_mesh"))
       buildLowerDMesh();
   }