Mk/bsd.sites.mk: Retire KDDI Research public FTP server

The annoucement page [1] will be available until August 15, 2025. See
also the article "History of KDDI Research's Public FTP Server" [2] for
the history and behind-the-scenes.

[1] https://ftp.kddilabs.jp/
[2] https://tech-note.kddi.com/n/n9a93ac6d5051

PR: 288399
(cherry picked from commit 5a115df579ac7d1a9f9d607d876da29a3e1d397d)