[S390] Remove P390 support.

Most likely it is broken anyway because of the changes in memory
detection. Since we can't test it and there are probably better ways
that using a P390 card, remove support for it.

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