s390/3270: drop unused spinlock from struct con3270

I was checking why this spinlock was never initialized, but it turns
out it's not used anywhere, so we can drop it.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>