catalog/externalcatalog: delete namespace entries during clean up

Previously, the external catalog logic would never clean up namespace
entries for descriptors when DropIngestedExternalCatalog was invoked.
This would lead to left over junk in the system.namespace table,
contrary to the normal object deletion protocol. To address this, this
patch also deletes namespace entries.

Fixes: #140551

Release note: None

Author: fqazi

pkg/sql/catalog/externalcatalog/external_catalog.go

@@ -256,6 +256,9 @@ func DropIngestedExternalCatalog(
 		if err := descsCol.WriteDescToBatch(ctx, kvTrace, t, b); err != nil {
 			return errors.Wrap(err, "writing dropping table to batch")
 		}
+		if err := descsCol.DeleteNamespaceEntryToBatch(ctx, kvTrace, t, b); err != nil {
+			return errors.Wrap(err, "writing namespace delete to batch")
+		}
 	}
 	return txn.KV().Run(ctx, b)
 }