x86 setup: drop SWAP_DEV

Impact: None (cleanup)

SWAP_DEV is unused since 2.6.23-rc1. The comment was already incorrect
since (at least) 2.6.12.

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