Variable "_custom_impl_clusters" unused before it went out of scope

When generating data model with no clusters then a build error is
generated due to the "_custom_impl_clusters" variable being unused
before it went out of scope.

```
ERROR at //src/app/chip_data_model.gni:192:29: Assignment had no effect.
    _custom_impl_clusters = []
                            ^
You set the variable "_custom_impl_clusters" here and it was unused
before it went out of scope.
```

To prevent the issue, "_custom_impl_clusters" can be marked as not
needed.

src/app/chip_data_model.gni

@@ -190,6 +190,7 @@ template("chip_data_model") {
     }
 
     _custom_impl_clusters = []
+    not_needed([ "_custom_impl_clusters" ])  # Might not be needed.
     if (defined(invoker.zap_clusters_with_custom_implementation)) {
       _custom_impl_clusters = invoker.zap_clusters_with_custom_implementation
     }