sandbox: u-boot.lds: Remove bogus __bss_start symbol

The sections described in the sandbox linker script are inserted before
data section via "INSERT BEFORE .data;". Running readelf -S on sandbox
u-boot binary shows that the bss section is located after the data
section:

  Section Headers:
  [Nr] Name              Type             Address           Offset
       Size              EntSize          Flags  Link  Info  Align
  ...
  [25] .u_boot_list      PROGBITS         000000000041d1c8  0021d1c8
       000000000000dd90  0000000000000000  WA       0     0     8
  [26] _u_boot_sandbox_g PROGBITS         000000000042af58  0022af58
       00000000000000a0  0000000000000000  WA       0     0     8
  [27] .data             PROGBITS         000000000042b000  0022b000
       000000000000f708  0000000000000000  WA       0     0     32
  [28] .bss              NOBITS           000000000043a720  0023a708
       0000000000018930  0000000000000000  WA       0     0     32

This means that the __bss_start assignment in the linker script is bogus,
as the actual bss section start is located elsewhere. Remove this
assignment, as the __bss_start symbol is not used on sandbox anyway.

Signed-off-by: Ovidiu Panait <ovidiu.panait@windriver.com>