perf machine: Move machine__remove_thread and make static

As the now only user, machine__process_exit_event, that is what tools
use to process PERF_RECORD_EXIT events, is on the same object file.

Signed-off-by: David Ahern <dsahern@gmail.com>
Link: http://lkml.kernel.org/r/1363151248-16674-5-git-send-email-dsahern@gmail.com


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