x86 setup: remove DEF_INITSEG and DEF_SETUPSEG

Since v.2.6.23 DEF_INITSEG and DEF_SETUPSEG are unused. Commit c3973682
("Remove old i386 setup code") dropped their usage for i386. They did not
return in the x86 tree. (Something similar must have happened for x86_64.)
Remove these.

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