Remove write quorum

The write quorum is saved in the index. We cosmetically remove it.

Fixes:COUCHDB-2663
diff --git a/src/mango_idx.erl b/src/mango_idx.erl
index bf81679..c886ab0 100644
--- a/src/mango_idx.erl
+++ b/src/mango_idx.erl
@@ -316,6 +316,8 @@
     filter_opts(Rest);
 filter_opts([{type, _} | Rest]) ->
     filter_opts(Rest);
+filter_opts([{w, _} | Rest]) ->
+    filter_opts(Rest);
 filter_opts([Opt | Rest]) ->
     [Opt | filter_opts(Rest)].