ports-mgmt/pkg_cleanup: update man page

The pkg_cleanup(1) man page refers to the obsolete man pages
pkg-delete(1) and pkg-info(1). These were part of the origin pkg
implementation which is long gone. These are now subcommands of the pkg
command and spelled "pkg delete" and "pkg info", but they have their own
man pages, pkg-delete.1 and pkg-info.1.

PR: 259955
Reported by: Graham Perrin <grahamperrin@gmail.com>