MINOR: tcp: avoid confusion in time parsing init

MINOR: tcp: avoid confusion in time parsing init

We never enter val_fc_time_value when an associated fetcher such as `fc_rtt` is
called without argument.  meaning `type == ARGT_STOP` will never be true and so
the default `data.sint = TIME_UNIT_MS` will never be set.  remove this part to
avoid thinking default data.sint is set to ms while reading the code.

Signed-off-by: William Dauchy <w.dauchy@criteo.com>

[Cf: This patch may safely backported as far as 1.7. But no matter if not.]