Remove all authors and permlinks from worker objects

[afalaleev]

After implementing of #1123 we don't need the following fields in worker objects:

class worker_proposal_object : public object<worker_proposal_object_type, worker_proposal_object> {
.....
        account_name_type author;
        account_name_type approved_techspec_author;
        shared_string approved_techspec_permlink;
....
 }

class worker_techspec_object : public object<worker_techspec_object_type, worker_techspec_object> {
 ....
        account_name_type author;
....
        account_name_type worker_proposal_author;
        shared_string worker_proposal_permlink;
....
        shared_string worker_result_permlink;
....
};