Remove remaining references to model.NewXX (#28678)

And we remove the deprecated functions to force uses
only of the new function.
```release-note
NONE
```

server/cmd/mmctl/commands/sampledata_util.go

@@ -398,7 +398,7 @@ func createDirectChannel(members []string) imports.LineImportData {
 
 	for _, m := range members {
 		p = append(p, &imports.DirectChannelMemberImportData{
-			Username: model.NewString(m),
+			Username: model.NewPointer(m),
 		})
 	}