Treat deleted security docs the same as missing

The same logic to migrate new security docs works as well for deleted docs.
diff --git a/src/cassim_metadata_cache.erl b/src/cassim_metadata_cache.erl
index 12f2a06..002ee43 100644
--- a/src/cassim_metadata_cache.erl
+++ b/src/cassim_metadata_cache.erl
@@ -198,7 +198,7 @@
         {ok, {ok, #doc{}=Doc}} ->
             couch_doc:to_json_obj(Doc, []);
         {ok, {not_found, deleted}} ->
-            deleted;
+            undefined;
         {ok, {not_found, missing}} ->
             undefined;
         {error, {database_does_not_exist, _}} ->