freshclam.conf.in: remove ReceiveTimeout option

freshclam.conf.in: remove ReceiveTimeout option

mirroring the changed default both in upstream [0] and debian [1].
Additionally since the upgrade to clamav 0.102.2 we had a number of reports
from users (both in forum and in our enterprise support), which noticed
that the upgrades don't work due to running into the timeout
(might be related to the use of libcurl in freshclam in 0.102.0)

absence of 'ReceiveTimeout' in freshclam.conf (5)  defaults to 0 (no timeout).

[0] https://blog.clamav.net/2020/02/clamav-01022-security-patch-released.html
[1] apt changelog clamav-freshclam (for version 0.102.2+dfsg-1)

Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>