Remove incorrect warning from pg_archivecleanup document.

Remove incorrect warning from pg_archivecleanup document.

The .backup file name can be passed to pg_archivecleanup even if
it includes the extension which is specified in -x option.
However, previously the document incorrectly warned a user
not to do that.

Back-patch to 9.2 where pg_archivecleanup's -x option and
the warning were added.