(#3211) - perf: remove unused metadata.seq idb index

lib/adapters/idb.js

diff --git a/lib/adapters/idb.js b/lib/adapters/idb.js
index 39b831d..e90e7da 100644
--- a/lib/adapters/idb.js
+++ b/lib/adapters/idb.js
@@ -270,7 +270,6 @@
   // called when creating a fresh new database
   function createSchema(db) {
     var docStore = db.createObjectStore(DOC_STORE, {keyPath : 'id'});
-    docStore.createIndex('seq', 'seq', {unique: true});
     db.createObjectStore(BY_SEQ_STORE, {autoIncrement: true})
       .createIndex('_doc_id_rev', '_doc_id_rev', {unique: true});
     db.createObjectStore(ATTACH_STORE, {keyPath: 'digest'});