Add 'arcyd rm-repo' for removing repos

Introduce a command for symmetry with 'arcyd add-repo' for when we
want to stop watching a repository.

Test Plan:
$ ./precommit.sh
$ ./testbed/arcyd/test_shell.sh
  $ cd ../arcyd
  $ arcyd rm-repo localhost_fs_origin
  .. fail because arcyd is running ..
  $ arcyd stop
  $ arcyd rm-repo localhost_fs_origin
  .. success, verify files no longer exist ..
  $ arcyd fsck
  $ arcyd rm-repo localhost_fs_origin
  .. fail, doesn't exist ..