mp_row_consumer: remove unnecessary _reasource_tracker member
mp_row_consumer: remove unnecessary _reasource_tracker member

Leftovers from a43901f84.

Signed-off-by: Botond Dénes <a584fa4d75a0a85d721f44fb6e06315a2853b0b6@scylladb.com>
Message-Id: <88237d9cd97feeca47e12ec4af89c90f1a3a6bb5.1507535176.git.a584fa4d75a0a85d721f44fb6e06315a2853b0b6@scylladb.com>

avikivity/scylla,duarten/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,duarten/scylla,avikivity/scylla,duarten/scylla,scylladb/scylla,scylladb/scylla