hide tplink sdcard default value

[bmw]

while reviewing #1134 i noticed that installer tplink --help looked like:

--sdcard-path <SDCARD_PATH>
  For advanced users: Specify the path of the SD card to be mounted explicitly.
  
  The default (empty string) is to use whichever sdcard path the device would use natively to mount storage on. On most TP-Link this is /media/card, but on hardware versions 9+ this is /media/sdcard
  
  Only override this when the installer does not work on your hardware version, as otherwise your custom path may conflict with the builtin storage functionality.
  
  [default: ]

i think this [default: ] looks bad and is redundant with the help text. setting this attribute removes it from the help output

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