gen_initramfs_list.sh: remove unused variable 'default_list'

This is assigned, but not referenced.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Greg Thelen <gthelen@google.com>

usr/gen_initramfs_list.sh

@@ -280,7 +280,6 @@ while [ $# -gt 0 ]; do
 			shift
 			;;
 		"-d")	# display default initramfs list
-			default_list="$arg"
 			${dep_list}default_initramfs
 			;;
 		"-h")