An extra argument for the filename defining the extension script
location was present, aimed at being used for error reporting, but has
never been used.  This was around since extensions have been added in
d9572c4e.

Author: Yugo Nagata
Reviewed-by: Tatsuo Ishii
Discussion: https://postgr.es/m/20180907180504.1ff19e1675bb44a67e9c7ab1@sraoss.co.jp