perf/core: Remove unused perf_cgroup_event_cgrp_time() function

The function was added by commit e5d1367f ("perf: Add cgroup
support") in 2011 and hasn't been used since then. Removing it fixes the
following warning when building with Clang:

    kernel/events/core.c:696:19: error: unused function 'perf_cgroup_event_cgrp_time' [-Werror,-Wunused-function]
Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
Cc: Douglas Anderson <dianders@chromium.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20170523215132.189049-1-mka@chromium.orgSigned-off-by: Ingo Molnar <mingo@kernel.org>

kernel/events/core.c

@@ -925,11 +925,6 @@ static inline int is_cgroup_event(struct perf_event *event)
 	return 0;
 }
 
-static inline u64 perf_cgroup_event_cgrp_time(struct perf_event *event)
-{
-	return 0;
-}
-
 static inline void update_cgrp_time_from_event(struct perf_event *event)
 {
 }