Remove RWD from vfolder permissions

Currently we have RO (read-only), RW (read-write), and RWD (read-write-delete) permission levels for a vfolder.

> The above 3-level vfolder permissions are for _bind-mount_ permissions in containers, and **_different from_** vfolder RBAC which defines the access & manipulation privileges of vfolder objects in the [Backend.AI](http://Backend.AI) control plane.

The RWD concept was first introduced in circa 2017, and I had planned to implement it by blocking `unlink` syscalls in the jail.

However, it turns out to be a useless concept and we did not implement it until now, because it is impossible to distinguish the normal application pattern to modify existing files by cloning and removing files.

Let’s retire and remove the RWD permissions from our codebase and the UI as well.

For backward compatibility of the API, we can coalesce RWD to RW safely when accepting API parameters.

JIRA Issue: BA-505