perf tools: Remove redundant initialization of thread linkage members

A thread moves from a rb tree to a list, but can't be on both, because
those linkage members are in a union. This is leftover from when I was
debugging thread refcounting and had nuked that union.

It is harmless duplication, as RB_CLEAR_NODE() does again what
INIT_LIST_HEAD does.

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/n/tip-hmma9lmip6qlhzhgkhp9tzd1@git.kernel.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>