login: remove login reminder page.

Remove the login reminder page. The meaning of "login reminder" wasn't
clear (it's for recovering a forgotten username) and the functionality
was redundant. The password reset page can already be used to recover
forgotten usernames.

There was also a privacy leak, since the login reminder page could be
used to find out whether a given email is in use on Danbooru.

test/functional/maintenance/user/login_reminders_controller_test.rb

@@ -1,35 +0,0 @@
-require "test_helper"
-
-module Maintenance
-  module User
-    class LoginRemindersControllerTest < ActionDispatch::IntegrationTest
-      context "A login reminder controller" do
-        setup do
-          @user = create(:user)
-          @blank_email_user = create(:user, :email => "")
-          ActionMailer::Base.delivery_method = :test
-          ActionMailer::Base.deliveries.clear
-        end
-
-        should "render the new page" do
-          get new_maintenance_user_login_reminder_path
-          assert_response :success
-        end
-
-        should "deliver an email with the login to the user" do
-          post maintenance_user_login_reminder_path, params: {:user => {:email => @user.email}}
-          assert_equal(1, ActionMailer::Base.deliveries.size)
-        end
-
-        context "for a user with a blank email" do
-          should "fail" do
-            post maintenance_user_login_reminder_path, params: {:user => {:email => ""}}
-            @blank_email_user.reload
-            assert_equal(@blank_email_user.created_at.to_i, @blank_email_user.updated_at.to_i)
-            assert_equal(0, ActionMailer::Base.deliveries.size)
-          end
-        end
-      end
-    end
-  end
-end