LES: change the GetHelperTrieProofs message in the les4

[rjl493456442]

Currently, the GetHelperTrieProofs is supported by LES protocol so that the client can retrieve the helper tries and some auxiliary data with proof.

However, the auxiliary data definition is a little bit confusing. There are two auxiliary data supported:

• auxReq = 1: in this case, the trie root of the specific helper trie is returned
• auxReq = 2: in this case, the requested block header is returned.

This issue will purpose to remove the support for auxReq = 1. I can't imagine any useful scenario that only the trie root is returned(with no proof) for this kind of request.