Remove duplicate appending of "6" to network type

To distinct between IPv4 and IPv6 configuration, a "6" was added to the network configuration type if an IPv6 address should be configured. This is now duplicate, as with pull 13024 the same thing is already done prior, thus leading to a duplicate "6" at the end of the network config type, i.e. "static66".

plugins/providers/virtualbox/action/network.rb

@@ -299,8 +299,6 @@ module VagrantPlugins
             elsif ip.ipv6?
               options[:netmask] ||= 64
 
-              # Append a 6 to the end of the type
-              options[:type] = "#{options[:type]}6".to_sym
             else
               raise IPAddr::AddressFamilyError, 'unknown address family'
             end