refactor little-used helper interface (#27894)

There's no production code that expect this specific interface and I tried to
remove it because I missed it was only used in a single test. Generally it's a
good idea to define such interfaces close to their point of use, but in this
case it's not even worth defining an interface for. The "generic memdb" branch I
worked on a while back has a `Count` method we'll be able to use for this kind
of thing down the road, but for now let's remove this otherwise dead code.

Author: tgross

helper/iterator/iterator.go

@@ -8,7 +8,6 @@ import (
 	"testing"
 
 	"github.com/hashicorp/nomad/ci"
-	"github.com/hashicorp/nomad/helper/iterator"
 	"github.com/hashicorp/nomad/helper/pointer"
 	"github.com/hashicorp/nomad/helper/uuid"
 	"github.com/hashicorp/nomad/nomad/mock"
@@ -201,7 +200,10 @@ func TestScheduler_JobRegister_MemoryMaxHonored(t *testing.T) {
 				case "system", "sysbatch":
 					nodes, err := h.State.NodesByNodePool(nil, job.NodePool)
 					must.NoError(t, err)
-					expectedAllocCount = iterator.Len(nodes)
+					expectedAllocCount = 0
+					for raw := nodes.Next(); raw != nil; raw = nodes.Next() {
+						expectedAllocCount++
+					}
 				}
 				must.Len(t, expectedAllocCount, allocs)
 				alloc := allocs[0]