of/fdt: Remove duplicate memory clearing on FDT unflattening

Patch 9e401275, "of: fdt: fix memory initialization for expanded DT"
fixed incomplete clearing of memory when unflattening the device tree.
However the code was already clearing some of the memory, it just wasn't
doing so for all allocations. Now that the memory is cleared right at
the point of allocation, the memset after unflatten_dt_alloc() is
redundant. Remove it.

Signed-off-by: Grant Likely <grant.likely@linaro.org>
Acked-by: Wladislav Wiebe <wladislav.kw@gmail.com>