Fix removal of on-link prefix on windows with netsh

Fix removal of on-link prefix on windows with netsh

When adding / removing IPv6 routes, the host bits need to be zeroed or
netsh.exe will refuse to handle the route.

Commit a24dd2e31 changed the way this is done, breaking removal of
the on-link IPv6 prefix for windows + netsh.exe.

Fix by adding explicitly calling route_ipv6_clear_host_bits() from
delete_route_connected_v6_net().

Trac: #1003

Signed-off-by: Gert Doering <gert@greenie.muc.de>
Acked-by: Selva Nair <selva.nair@gmail.com>
Message-Id: <20180220160016.69786-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg16508.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>