s390/setup: cleanup reserve/remove_oldmem

Since OLDMEM_BASE/OLDMEM_SIZE is already taken into consideration and is
reflected in ident_map_size. reserve/remove_oldmem() is no longer needed
and could be removed.

Reviewed-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>