[S390] vdso: remove redundant check for CONFIG_64BIT

This is a check for CONFIG_64BIT inside a block that is only active when
CONFIG_64BIT is set. So the check is actually useless and potentially
irritating.

Signed-off-by: Christoph Egger <siccegge@cs.fau.de>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>