s390/compat: remove compat exec domain

The whole compat exec domain code doesn't make any difference.

From the registered s390_exec_domain:

- exec domain name is only displayed in /proc/execdomains
- handler is unused
- pers_low and pers_high are only used internally to find this specific
  exec domain otherwise the default exec domain will be used
- all other fields match the default exec domain

So let's get rid of this.

Reported-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>