perf db-export: No need to have ->thread twice in struct export_sample

perf db-export: No need to have ->thread twice in struct export_sample

As it comes from address_location->thread, that is already stored as
export_sample->al, where the thread can be obtained.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Borislav Petkov <bp@suse.de>
Cc: David Ahern <dsahern@gmail.com>
Cc: Don Zickus <dzickus@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/r/20150402141542.GA9630@kernel.org
Link: http://lkml.kernel.org/n/tip-bzotbl4epoztw0jd6sm2stpf@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>