vmlinux.lds.h: remove unused CPU_KEEP and CPU_DISCARD macros

Remove the left-over of commit e24f6628 ("modpost: remove all
traces of cpuinit/cpuexit sections").

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Acked-by: Paul Gortmaker <paul.gortmaker@windriver.com>