Arcyd: remove dep of on_io_event on arcydreporter

Make it possible to remove the arcydreporter by removing the last
critical dependency on it. abdt_logging.on_io_event() will now work
without it.

Test Plan:
$ ./precommit.sh

Check the log is still updated
$ ./testbed/arcyd/test_shell.sh
    $ ./poke.sh
    $ exit
    .. view last lines of output ..
Arcyd: BREAK, remove arcyd and repo reporters

Prior to moving to a multi-process model, start removing functionality
that won't fit cleanly into the new model.

Remove the 'arcydreporter' and 'reporeporter' code paths.

This breaks logging some performance stats via the external reporter
registered with `--external-report-command`.

Test Plan:
$ ./precommit.sh
$ git grep abdt_arcydreporter
$ git grep abdt_reporeporter