perf: Simplify task_clock_event_read()

There is no point in us having different code paths for nmi and !nmi
here, so remove the !nmi one.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Stephane Eranian <eranian@google.com>
LKML-Reference: <new-submission>
Signed-off-by: Ingo Molnar <mingo@elte.hu>