clamav: remove deprecated SafeBrowsing

clamav: remove deprecated SafeBrowsing

this database was discontinued in 2019 due to legal reasons.

see:
https://blog.clamav.net/2021/04/are-you-still-attempting-to-download.html

archive in case it goes 404:
https://web.archive.org/web/20210408140341/https://blog.clamav.net/2021/04/are-you-still-attempting-to-download.html

* remove from freshclam.conf.in template
* remove from description
* default to 0 just to be sure
* if 'safebrowsing' set in pmg.conf, this is now ignored

note about removing the option in PMG 7.0

Signed-off-by: Oguz Bektas <o.bektas@proxmox.com>
changed the removal notice form PMG 7.0 to 8.0
Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>