handbook: Drop duplicated URLs in Mirrors

Those were supposed to be separate URLs for IPv4 and IPv6,
but, well, they aren't. Drop the 'v6' ones; as it is now,
it's just duplicated URLs, and it looks weird and confusing.

Reviewed By: debdrup, Pau Amma
Sponsored By: EPSRC
Differential Revision: https://reviews.freebsd.org/D33982