[S390] Remove diag 0x260 call from memory detection.

The result of the diag 0x260 call is not always what one would expect.
So just remove it.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>