`-2` used to mean "Use global limits", which no longer exist since
3.17.0. As any negative value results in the auto bandwidth limiter to
engage, this could result in unexpected slower sync speeds after a
client upgrade.
Let's just keep handling that value and assume that we want to use
unlimited bandwidth.
Fixes#8743
Signed-off-by: Jyrki Gadinger <nilsding@nilsding.org>