boot, efi: Remove redundant memset()

Remove a redundant memset() call from efi_relocate_kernel() that
was clearing memory that would be used by BSS in non-compressed
images loaded with this function.  This clear was redundant with
the clearing done in the image itself, and also implemented incorrectly
with a 0 length.

Signed-off-by: Roy Franz <roy.franz@linaro.org>
Acked-by: Mark Salter <msalter@redhat.com>
Signed-off-by: Matt Fleming <matt.fleming@intel.com>