revert change to PendingEnvelopes::addSCPQuorumSet

change introduced in d1b33a4 was wrong
hash was the hash of a quorum set not the hash of a node

src/herder/PendingEnvelopes.cpp

@@ -66,16 +66,7 @@ PendingEnvelopes::addSCPQuorumSet(Hash hash, const SCPQuorumSet& q)
     CLOG(TRACE, "Herder") << "Add SCPQSet " << hexAbbrev(hash);
 
     SCPQuorumSetPtr qset(new SCPQuorumSet(q));
-    if (mQsetCache.exists(hash))
-    {
-        // force recomputation of transitive quorum information as it may change
-        // "not in quorum" into "in quorum".
-        // the "quorum -> not in quorum" is similar to the case of a new quorum
-        // set, where the only thing it can do is turn
-        // a "maybe" (true) into "no" (false) which doesn't matter within
-        // a round (clear will happens regardless when the slot is externalized)
-        mNodesInQuorum.clear();
-    }
+    mNodesInQuorum.clear();
     mQsetCache.put(hash, qset);
 
     mQuorumSetFetcher.recv(hash);