perf_counter tools: Remove dead code

Vince Weaver reported that there's a handful of #ifdef __MINGW32__
sections in the code.

Remove them as they are in essence dead code - as unlike upstream
Git, the perf tool is unlikely to be ported to Windows.

Reported-by: Vince Weaver <vince@deater.net>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
LKML-Reference: <new-submission>
Signed-off-by: Ingo Molnar <mingo@elte.hu>