perf session: Remove unused perf_session__remove_thread method

Should have been removed on this changeset, that removed the last user
of it:

  743eb868657bdb1b26c7b24077ca21c67c82c777

    perf tools: Resolve machine earlier and pass it to perf_event_ops

Signed-off-by: David Ahern <dsahern@gmail.com>
Link: http://lkml.kernel.org/r/1363151248-16674-4-git-send-email-dsahern@gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>