Fix unused local variable linting error

Author: Pierre Ayoub

beetsplug/convert.py

@@ -351,7 +351,7 @@ def convert_item(
             # to avoid any side-effect of the conversion (linking, keep_new,
             # refresh) if we already know that it will fail.
             try:
-                mf = mediafile.MediaFile(util.syspath(item.path))
+                mediafile.MediaFile(util.syspath(item.path))
             except mediafile.UnreadableFileError as exc:
                 self._log.error("Could not open file to convert: {0}", exc)
                 continue