pldm: Removing default string length

Removing default string length from BIOS json configuration
files.

String BIOS attributes defined in BIOS configuration json file
contains "default_string_length". Providing default string length
manually is error prone. As String length can be calculated and
populated easily in the code, removing json support for
default_string_length.

Tested: Power on/off
        Gtests passed.

Change-Id: I7604d8ccedd60a39ba07e1df67ff5712f31e7f6e
Signed-off-by: Archana Kakani <archana.kakani@ibm.com>