perf tests time-to-tsc: No need to disable an event before deleting it

perf tests time-to-tsc: No need to disable an event before deleting it

Because at the destructor we will call close() and that will do the
disable. And we destructors can accept NULL, just like free(), so no
need to check it.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Jiri Olsa <jolsa@redhat.com>
Link: http://lkml.kernel.org/n/tip-i98mcyfkkjh5qp62dle27ac1@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>