LES: change the GetHelperTrieProofs message in the les4

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.