Revert "drive: server side copy docs use default description if empty"

This reverts commit 9e4b68a364.

This does not work as intended - it only changes docs files and to
make it change drive files would take an extra roundtrip.

I think the sematics of server side copy are now correct - additional
features should be added with a new flag.

See #4230

backend/drive/drive.go

@@ -2244,10 +2244,7 @@ func (f *Fs) Copy(ctx context.Context, src fs.Object, remote string) (fs.Object,
 		if err != nil {
 			return nil, errors.Wrap(err, "failed to read description for Google Doc")
 		}
-		// set the description if there is one, or use the default if not
-		if info.Description != "" {
-			createInfo.Description = info.Description
-		}
+		createInfo.Description = info.Description
 	} else {
 		// don't overwrite the description on copy for files
 		// this should work for docs but it doesn't - it is probably a bug in Google Drive