Remove duplication in deploy environment files

Deploy environment files were loaded after config/deploy.rb was loaded,
meaning if we used settings like `deploy_to` or `current_path` to set
variables inside the config/deploy.rb file, we'd be using the default
settings instead of the ones dictated by the deploy environment files.

config/deploy/staging.rb

@@ -1,7 +1,3 @@
-set :deploy_to, deploysecret(:deploy_to)
-set :server_name, deploysecret(:server_name)
-set :db_server, deploysecret(:db_server)
 set :branch, ENV["branch"] || :master
-set :ssh_options, port: deploysecret(:ssh_port)
 
 server deploysecret(:server), user: deploysecret(:user), roles: %w[web app db importer cron]