Don't print default values for arguments twice

$ sbws generate -h
[... snip ...]
  -m TORFLOW_BW_MARGIN, --torflow-bw-margin TORFLOW_BW_MARGIN
                        Cap maximum bw when scaling as Torflow. (Default:
                        0.05) (default: 0.05)
  -r TORFLOW_ROUND_DIGS, --torflow-round-digs TORFLOW_ROUND_DIGS
                        Number of most significant digits to round bw when
                        scaling as Torflow. (Default: 3) (default: 3)
  -p SECS_RECENT, --secs-recent SECS_RECENT
                        How many secs in the past are results being still
                        considered. Note this value will supersede data_period
                        in the configuration. (Default: None) (default: None)
  -a SECS_AWAY, --secs-away SECS_AWAY
                        How many secs results have to be away from each other.
                        (Default: 86400 - one day -) (default: 86400)
  -n MIN_NUM, --min-num MIN_NUM
                        Mininum number of a results to consider them.(Default:
                        2) (default: 2)