CORD-1465 Assume no map_delete_inputs means an empty delete sync step

Change-Id: Icbfc0801a6f83f6c4540fa11a9c92c1da12112af
diff --git a/xos/synchronizers/new_base/syncstep.py b/xos/synchronizers/new_base/syncstep.py
index 34b112f..b264f5a 100644
--- a/xos/synchronizers/new_base/syncstep.py
+++ b/xos/synchronizers/new_base/syncstep.py
@@ -166,6 +166,10 @@
         #        except AttributeError:
         #            pass
 
+        # If there is no map_delete_inputs, then assume deleting a record is a no-op.
+        if not hasattr(self, "map_delete_inputs"):
+            return
+
         tenant_fields = self.map_delete_inputs(o)
 
         main_objs = self.observes