Add thread_map__remove function to remove thread from thread map.

Add automated test also.

Committer notes:

Testing it:

  # perf test "Remove thread map"
  39: Remove thread map                          : Ok
  # perf test -v "Remove thread map"
  39: Remove thread map                          :
  --- start ---
  test child forked, pid 4483
  2 threads: 4482, 4483
  1 thread: 4483
  0 thread:
  test child finished with 0
  ---- end ----
  Remove thread map: Ok
  #
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1481538943-21874-4-git-send-email-jolsa@kernel.org
[ Added stdlib.h, to get the free() declaration ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>