Remove whitespace from authentication generator templates (rails#52469)

These extra lines of whitespace cause failures from Rubocop's
Layout/TrailingEmptyLines rule which is enabled by default in Rails at the time
of writing.

As such creating a brand new Rails app and then running `rails generate
authentication` will give you Rubocop failures.

Author: markjs

railties/lib/rails/generators/rails/authentication/templates/controllers/concerns/authentication.rb

@@ -61,4 +61,3 @@ def terminate_session
       cookies.delete(:session_token)
     end
 end
-

railties/lib/rails/generators/rails/authentication/templates/models/current.rb

@@ -2,4 +2,3 @@ class Current < ActiveSupport::CurrentAttributes
   attribute :session
   delegate :user, to: :session, allow_nil: true
 end
-

railties/lib/rails/generators/rails/authentication/templates/models/session.rb

@@ -2,4 +2,3 @@ class Session < ApplicationRecord
   has_secure_token
   belongs_to :user
 end
-