storage/reflink: volume usage(): omit wrong docstring

It's neither correct to say that this is "volume disk usage from the
VM's perspective" (because an image file can be sparsified on the host,
e.g. by restoring a backup, cloning a VM, or 'fallocate --dig-holes')
nor that disk usage is "usually much lower from the host's perspective"
(because it's not unusual for a volume to have multiple image files with
lots of diverging data, e.g. revisions).

Hard to say anything there that's true, helpful, and not a novella.

qubes/storage/reflink.py

@@ -348,9 +348,6 @@ def size(self):
 
     @property
     def usage(self):
-        ''' Return volume disk usage from the VM's perspective. It is
-            usually much lower from the host's perspective due to CoW.
-        '''
         for path in (self._path_dirty, self._path_clean):
             with suppress(FileNotFoundError):
                 return os.stat(path).st_blocks * 512