Removes vagrant-hostmanager plugin from Vagrantfile (#1311)

The plugin is not used in the config anywhere, nor is the requirement
documented in our developer docs. Therefore let's remove it, because
first-time setup for SecureDrop VMs is a frustrating experience when
it's not nearly as simple as `vagrant up`.

Vagrantfile

@@ -46,7 +46,6 @@ Vagrant.configure("2") do |config|
     staging.vm.hostname = "mon-staging"
     staging.vm.box = "trusty64"
     staging.vm.network "private_network", ip: "10.0.1.3", virtualbox__intnet: true
-    staging.hostmanager.aliases = %w(securedrop-monitor-server-alias)
     staging.vm.box_url = "https://cloud-images.ubuntu.com/vagrant/trusty/current/trusty-server-cloudimg-amd64-vagrant-disk1.box"
     staging.vm.synced_folder './', '/vagrant', disabled: true
     staging.vm.provider "virtualbox" do |v|