Do not create postgresql index on an inexisting table

This table was deleted, this leftover index was preventing from starting
a new installation with postgresql.

Change-Id: I10a74ced51c88562249e0e37d4d71cc07d092710
diff --git a/resources/com/google/gerrit/reviewdb/server/index_postgres.sql b/resources/com/google/gerrit/reviewdb/server/index_postgres.sql
index f2f24e1..439fed7 100644
--- a/resources/com/google/gerrit/reviewdb/server/index_postgres.sql
+++ b/resources/com/google/gerrit/reviewdb/server/index_postgres.sql
@@ -8,7 +8,6 @@
 ALTER TABLE patch_comments CLUSTER ON patch_comments_pkey;
 ALTER TABLE patch_set_approvals CLUSTER ON patch_set_approvals_pkey;
 
-ALTER TABLE account_group_members CLUSTER ON account_group_members_pkey;
 CLUSTER;