Use only one server when deploying to preproduction

It doesn't make much sense that by default we use one server on
production on two servers on preproduction.

Note we're keeping `server1` instead of using just `server` in order to
keep compatibility with existing installation.

config/deploy-secrets.yml.example

@@ -8,7 +8,6 @@ preproduction:
   deploy_to: "/home/deploy/consul"
   ssh_port: "2222"
   server1: xxx.xxx.xxx.xxx
-  server2: xxx.xxx.xxx.xxx
   user: "deploy"
 
 production:

config/deploy/preproduction.rb

@@ -1,4 +1,3 @@
 set :branch, ENV["branch"] || :master
 
 server deploysecret(:server1), user: deploysecret(:user), roles: %w[web app db importer cron background]
-server deploysecret(:server2), user: deploysecret(:user), roles: %w[web app db importer]