s390/debug: remove struct __debug_entry from uapi

There is no interface to userspace which exposes anything that would
require the struct __debug_entry definition. Therefore remove it from
uapi. This allows to change the definition, since it is only kernel
internally used.

The only exception is the crash utility, however that tool must handle
changes all the time anyway.

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