perf header: Delete an unnecessary check before the calling free_event_desc()

The free_event_desc() function tests whether its argument is NULL and
then returns immediately. Thus the test around the call is not needed.

This issue was detected by using the Coccinelle software.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Cc: Julia Lawall <julia.lawall@lip6.fr>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: kernel-janitors@vger.kernel.org
Link: http://lkml.kernel.org/r/558C2ABA.3000603@users.sourceforge.net
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>