unify session destroy

app/controllers/application_controller.rb

@@ -62,4 +62,9 @@ class ApplicationController < ActionController::Base
     session[:expires_at] = 15.minutes.from_now.to_i
   end
   helper_method :update_session_expiry
+  private
+  def destroy_session
+    session[:user_id] = nil
+    session[:trees_recovery_token] = nil
+  end
 end

app/controllers/delete_account_controller.rb

@@ -4,8 +4,9 @@ class DeleteAccountController < ApplicationController
       begin
         ApiBackend::delete_account(current_user, p)
         flash[:notice] = :delete_success
-        session[:user_id] = nil
+        destroy_session
         redirect_to '/'
+        return
       rescue ApiBackend::ApiError => e
         if e.api_msg == 'auth_fail'
           flash[:notice] = :auth_fail

app/controllers/sessions_controller.rb

@@ -58,8 +58,7 @@ class SessionsController < ApplicationController
   end
 
   def destroy
-    session[:user_id] = nil
-    session[:trees_recovery_token] = nil
+    destroy_session
     redirect_to '/login'
   end