ref-filter: drop useless cast in trailers_atom_parser()

There's no need to cast invalid_arg before freeing it. It is already a
non-const pointer.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

ref-filter.c

@@ -578,7 +578,7 @@ static int trailers_atom_parser(struct ref_format *format UNUSED,
 				strbuf_addf(err, _("expected %%(trailers:key=<value>)"));
 			else
 				strbuf_addf(err, _("unknown %%(trailers) argument: %s"), invalid_arg);
-			free((char *)invalid_arg);
+			free(invalid_arg);
 			return -1;
 		}
 	}