libfdt: simplify fdt_del_mem_rsv()

The variable "err" is unneeded.

[ Device Tree Compiler commit: 36fd7331fb11276c09a6affc0d8cd4977f2fe100 ]

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Acked-by: Simon Glass <sjg@chromium.org>