[S390] remove ieee_instruction_pointer from thread_struct

The ieee_instruction_pointer can not be read from user space anymore
since git commit 613e1def, the ptrace
interface always returns zero. Remove it from the thread_struct. It
is still present in the user_regs_struct for compatability reasons.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>