htpdate: stop sending User-Agent that fakes Tor Browser

See “[Tails-dev] Faking htpdate user agent worth it?” (December 2016).

• config/chroot_local-includes/etc/default/htpdate.user-agent must go
• Corresponding doc updates: git grep -F /etc/default/htpdate.user-agent

Feature Branch: bugfix/17143-17214-12023-remove-obsolete-tor-browser-customization+force-all-tests

Related issues

• Related to #17233
• Related to #11562
• Blocked by #17143 (closed)

Original created by @intrigeri on 12023 (Redmine)