www/squid: Update to 6.4

Changelog: https://github.com/squid-cache/squid/blob/7254f20cbc449119ae39bfeeced965f2c80bad75/ChangeLog

Also remove a dead mirror.

PR: 274646
Approved by: tcberner (mentor)
Differential Revision: https://reviews.freebsd.org/D42324