perf probe: Remove unused tracing_dir variable

Remove unused tracing_dir variable from open_probe_events().

Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/147201827792.5713.4165387506020511920.stgit@devbox
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>