board: sifive: remove the command for setting serial number

We wouldn't like to allow user to change the serial number, so remove
the command for changing serial number in EEPROM.

Signed-off-by: Zong Li <zong.li@sifive.com>
Suggested-by: David Abdurachmanov <david.abdurachmanov@sifive.com>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>