[S390] sclp_tty: remove ioctl interface.

After all we came to the conclusion that this interface doesn't make any
sense. Besides that the ioctl number used was never registered, the header
file isn't exported, and we doubt there is even a single user.
So remove this interface, since it eases maintenance.

Cc: Peter Oberparleiter <peter.oberparleiter@de.ibm.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>