fvp: Remove call to bl2_get_ns_mem_layout() function

On FVP platforms, for now it is assumed that the normal-world
bootloader is already sitting in its final memory location.
Therefore, BL2 doesn't need to load it and so it doesn't need
to know the extents of the non-trusted DRAM.

Change-Id: I33177ab43ca242edc8958f2fa8d994e7cf3e0843