Revert "perf: Fix exit() vs PERF_FORMAT_GROUP"

This reverts commit 4fd38e45.

It causes various crashes and hangs when events are activated.

The cause is not fully understood yet but we need to revert it
because the effects are severe.

Reported-by: Stephane Eranian <eranian@google.com>
Reported-by: Lin Ming <ming.m.lin@intel.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Corey Ashford <cjashfor@linux.vnet.ibm.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
LKML-Reference: <new-submission>
Signed-off-by: Ingo Molnar <mingo@elte.hu>