[Disk Manager] Refactor code for resource creation/deletion.

[gy2411]

When we delete resource (disk, snapshot, etc), we use the following pattern. If the resource does not exist in dm database, then we create 'zero' object representing this resource and then proceed as if it was found in database (example for disks). This was needed to delete resources that are not known for disk-manager (for example, snapshots from legacy snapshot storage).

Now this seems to be unnecessary. Here #2497 we get rid of this pattern for snapshots and images. Need to get rid of this for all other types of resources.

Warning! Before refactoring code for some type of resource, make sure that there are no resources of this type that exist for blockstore but don't exist for disk manager.