fix(Vagrantfile): remove override-plugin

As of CoreOS 310.0.0, using the Vagrantfile results in the following
warning:

```
/Users/carmstrong/.vagrant.d/boxes/coreos-alpha/0/virtualbox/configure_networks.rb:13: warning: already initialized constant BASE_CLOUD_CONFIG
/Users/carmstrong/code/coreos-vagrant/override-plugin.rb:12: warning: previous definition of BASE_CLOUD_CONFIG was here
/Users/carmstrong/.vagrant.d/boxes/coreos-alpha/0/virtualbox/configure_networks.rb:25: warning: already initialized constant NETWORK_UNIT
/Users/carmstrong/code/coreos-vagrant/override-plugin.rb:36: warning: previous definition of NETWORK_UNIT was here
CoreOS (alpha)
core@core-01 ~ $
```

This version of CoreOS bundles its own configure_networks, making override-plugin unnecessary.
This commit removes override-plugin.