Postprocessors: use a list for the files that can be deleted

We could only know if we had to delete the original file, but this system allows to specify us more files (like subtitles).

youtube_dl/postprocessor/xattrpp.py

@@ -105,8 +105,8 @@ class XAttrMetadataPP(PostProcessor):
                     byte_value = value.encode('utf-8')
                     write_xattr(filename, xattrname, byte_value)
 
-            return True, info
+            return [], info
 
         except (subprocess.CalledProcessError, OSError):
             self._downloader.report_error("This filesystem doesn't support extended attributes. (You may have to enable them in your /etc/fstab)")
-            return False, info
+            return [], info