Turn off statement based binlog checks

From contributor PR https://github.com/percona/percona-toolkit/pull/193

For RDS we use row based replication however pt-table-checksum wants
to enforce statement and despite passing in the
--no-check-binlog-format flag, it would still run it (even though the
documentation suggests it shouldn't). This appears to be a bug
because there is another binlog format check that is wrapped in
$o->get('check-binlog-format') slightly further down.

To resolve this, I've updated the remaining binlog format check in the
same expression as the one below and it now honours the
--no-check-binlog-format as expected.