perf/doc: Remove mention of non-existent set_perf_event_pending() from design.txt

perf/doc: Remove mention of non-existent set_perf_event_pending() from design.txt

set_perf_event_pending() was removed in e360adbe ("irq_work: Add
generic hardirq context callbacks").

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Arnaldo Carvalho de Melo <acme@ghostprotocols.net>
Link: http://lkml.kernel.org/r/4c54761865d40210be0628cb84701afc5d57b5d8.1390686193.git.baruch@tkos.co.il
Signed-off-by: Ingo Molnar <mingo@kernel.org>