s390/Kconfig: Select VMAP_STACK unconditionally

s390/Kconfig: Select VMAP_STACK unconditionally

There is no point in supporting !VMAP_STACK kernel builds. VMAP_STACK has
proven to work since many years. Also, since KASAN_VMALLOC is supported,
kernels built with !VMAP_STACK are completely untested.

Therefore select VMAP_STACK unconditionally and remove all config options
and code required for !VMAP_STACK builds.

Acked-by: Christian Borntraeger <borntraeger@linux.ibm.com>
Reviewed-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>