s390/uaccess: remove copy_from_user_real()

There is no user left, so remove it.
It was also potentially broken, since the function didn't clear destination
memory if copy_from_user() failed. Which would allow for information leaks.

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