projectsmirrors: remove kernel mirror ftp.riken.jp

This mirror has not been synchronized for over a year.

Signed-off-by: Shiji Yang <yangshiji66@outlook.com>
Link: openwrt#19509
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>

Author: DragonBluep

scripts/projectsmirrors.json

@@ -49,7 +49,6 @@
 		"https://mirrors.mit.edu/kernel",
 		"http://ftp.nara.wide.ad.jp/pub/kernel.org",
 		"http://www.ring.gr.jp/archives/linux/kernel.org",
-		"https://ftp.riken.jp/Linux/kernel.org",
 		"https://www.mirrorservice.org/sites/ftp.kernel.org/pub",
 		"https://mirrors.ustc.edu.cn/kernel.org"
 	],