riak_cs_access_archiver_manager always fail to throttle even when
number of active workers exceeded max_workers, because the
handle_call({archive, _, _}, ...) never matches to the message
called from riak_cs_access_archiver_manager:archive/2.
Remove name reference to access archiver processes

Original issue: #746

To complete multiple archiver processes support introduced at:
  #717 Only a single access archiver can run at a time
  https://github.com/basho/riak_cs/pull/717
registered name references should be removed.
add xref fix
Merge pull request #758 from basho/bugfix/access-archiver-throttle

riak_cs_access_archiver_manager always fails to throttle access archiver