Remove hosts-file.net/ad_servers.txt (#1791)

hosts-file.net/ad_servers.txt now leads to a 404 page. The list is no longer published nor maintained.

More information:

https://forums.malwarebytes.com/topic/257401-inquiry-regarding-automated-processing-of-hosts-files/
https://github.com/pi-hole/pi-hole/pull/3236#issue-396455876
https://github.com/uBlockOrigin/uBlock-issues/issues/971#issue-591298291
https://www.reddit.com/r/pihole/comments/fsg11e/hostsfilenet/

config.cfg

@@ -49,7 +49,6 @@ reduce_mtu: 0
 # /etc/systemd/system/dnsmasq.service.d/100-CustomLimitations.conf
 adblock_lists:
  - "https://raw.githubusercontent.com/StevenBlack/hosts/master/hosts"
- - "https://hosts-file.net/ad_servers.txt"
 
 # Enable DNS encryption.
 # If 'false', 'dns_servers' should be specified below.