explore/posts/intro: remove page.

This was an alternate frontpage that contained a list of previews of the
most popular tags. This page was never linked from anywhere and it was
unknown by most users.

test/unit/post_sets/intro_test.rb

@@ -1,40 +0,0 @@
-require 'test_helper'
-
-module PostSets
-  class IntroTest < ActiveSupport::TestCase
-    context "In all cases" do
-      setup do
-        @user = FactoryBot.create(:user)
-        CurrentUser.user = @user
-        CurrentUser.ip_addr = "127.0.0.1"
-
-        @post_1 = FactoryBot.create(:post, :tag_string => "a")
-        @post_2 = FactoryBot.create(:post, :tag_string => "b")
-        @post_3 = FactoryBot.create(:post, :tag_string => "c")
-      end
-
-      teardown do
-        CurrentUser.user = nil
-        CurrentUser.ip_addr = nil
-      end
-
-      context "a set for the 'a' tag query" do
-        setup do
-          @post_4 = FactoryBot.create(:post, :tag_string => "a", :fav_count => 5)
-          @post_5 = FactoryBot.create(:post, :tag_string => "a", :fav_count => 5)
-        end
-
-        context "with no page" do
-          setup do
-            @set = PostSets::Intro.new("a")
-            ::Post.stubs(:records_per_page).returns(1)
-          end
-
-          should "return the first element" do
-            assert_equal(@post_5.id, @set.posts.first.id)
-          end
-        end
-      end
-    end
-  end
-end