MINOR: http: release compression context only in http_end_txn()

MINOR: http: release compression context only in http_end_txn()

Currently there are two places where the compression context is released,
one in session_free() and another one in http_end_txn_clean_session().
Both of them call http_end_txn(), either directly or via http_reset_txn(),
and this function is made for this exact purpose. So let's centralize the
call there instead.