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.ilSigned-off-by: Ingo Molnar <mingo@kernel.org>

tools/perf/design.txt

@@ -454,7 +454,6 @@ So to start with, in order to add HAVE_PERF_EVENTS to your Kconfig, you
 will need at least this:
 	- asm/perf_event.h - a basic stub will suffice at first
 	- support for atomic64 types (and associated helper functions)
-	- set_perf_event_pending() implemented
 
 If your architecture does have hardware capabilities, you can override the
 weak stub hw_perf_event_init() to register hardware counters.