MEDIUM: http: remove the now useless http_txn from {req/res} rules

MEDIUM: http: remove the now useless http_txn from {req/res} rules

The registerable http_req_rules / http_res_rules used to require a
struct http_txn at the end. It's redundant with struct stream and
propagates very deep into some parts (ie: it was the reason for lua
requiring l7). Let's remove it now.