Worker host information remains after detached

## Repro steps:
1. create a worker, put a host file in info folder.
2.connect the worker to buildbot, see the host info is shown in Workers page.
3. disconnect the worker, the host info is still shown in workers page.

This behavior is ok for static workers. However, it is especially annoying for Latent workers, as it shows staled host information.

## Analysis:
From the code, I can see the bot information are filled In AbstractWorker.attached callback.
But there's no way to clear the bot information in AbstractWorker.detached callback.

Could you advise how can I clear the bot information when the worker is detached? Thanks!