spl: spl_relocate_stack_gd: Do not unnecessarily clear bss

spl_relocate_stack_gd only gets called from arch/arm/lib/crt0.S which
clears the bss directly after calling it, so there is no need to clear
it from spl_relocate_stack_gd.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Simon Glass <sjg@chromium.org>