Fixes#6207.
As noted by Erica:
- we no longer need to check if it exists before linking to it, because we delete properly.
- the previously excisting check on if server is in `LE_REUSE_SERVERS` before unlinking is nice, but probably not necessary, especially since we don't officially support people doing weird things with symlinks in our directories, and because we rmdir which will fail if it's not empty anyway.
* Create single account symlink.
* refactor _delete_accounts_dir_for_server_path
* add symlinked account dir deletion
* add tests
(cherry picked from commit 9b0d2714c1)