Remove privileged-mounts setting

**What are you trying to do?**
Now that clients need to be authenticated with the Multipass daemon, and since no admin role has been needed for a while to set daemon settings, I don't see a reason to keep privileged-mounts disabled by default on Windows, nor to keep the setting at all.

**What's your proposed solution?**
Remove the setting and always allow privileged mounts to authenticated users.

The issue names no files or tests; begin by locating the privileged-mounts setting and the daemon's authenticated-client and mount authorization paths. Confirm where the Windows default is defined, remove the setting from configuration and documentation surfaces, and verify that authenticated users can always use privileged mounts.

Written by the indexing model from the issue text.