[S390] kprobes: single stepped breakpoint

Remove special case of a kprobe on a breakpoint while a relocated
instruction is single stepped. The only instruction that may cause
a fault while kprobe single stepping is active is the relocated
instruction. There is no kprobe on the instruction slot retrieved
with get_insn_slot().

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