perf events: Remove unused perf_counter.h header file

Since nothing includes the <linux/perf_counter.h> file and it's
also not exported to user space, remove it.
Signed-off-by: Robert P. J. Day <rpjday@crashcourse.ca>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
LKML-Reference: <alpine.LFD.2.00.0912161007430.8198@localhost>
Signed-off-by: Ingo Molnar <mingo@elte.hu>