x86: image: drop unused ROOTDELAY variable

It's unused since commit 74270071 ("x86: remove the olpc subtarget,
it has been unmaintained for a long time").

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>

target/linux/x86/image/Makefile

@@ -97,8 +97,6 @@ ifneq ($(CONFIG_GRUB_IMAGES),)
   endef
 endif
 
-ROOTDELAY=10
-
 define Image/Build/squashfs
 	dd if=/dev/zero bs=128k count=1 >> $(KDIR)/root.squashfs
 endef