nixos/networkmanager: remove networking.networkmanager.dynamicHosts

This option was removed because allowing (multiple) regular users to
override host entries affecting the whole system opens up a huge attack
vector. There seem to be very rare cases where this might be useful.
Consider setting system-wide host entries using networking.hosts,
provide them via the DNS server in your network, or use
networking.networkmanager.appendNameservers to point your system to
another (local) nameserver to set those entries.