x86 setup: remove IMAGE_OFFSET

After commit 968de4f0 ("i386: Relocatable kernel support") IMAGE_OFFSET wasn't
actually used anymore in the (current) X86 build system. Now remove its last
traces.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>