tags: remove Tag.trending method.

This is used for the frontpage taglist when popular searches are
disabled. This isn't used in production and there's no need for it in
development.

test/unit/tag_test.rb

@@ -12,24 +12,6 @@ class TagTest < ActiveSupport::TestCase
     CurrentUser.ip_addr = nil
   end
 
-  context ".trending" do
-    setup do
-      Tag.stubs(:trending_count_limit).returns(0)
-
-      travel_to(1.week.ago) do
-        FactoryBot.create(:post, :tag_string => "aaa")
-        FactoryBot.create(:post, :tag_string => "bbb")
-      end
-
-      FactoryBot.create(:post, :tag_string => "bbb")
-      FactoryBot.create(:post, :tag_string => "ccc")
-    end
-
-    should "order the results by the total post count" do
-      assert_equal(["ccc", "bbb"], Tag.trending)
-    end
-  end
-
   context "A tag category fetcher" do
     should "fetch for a single tag" do
       FactoryBot.create(:artist_tag, :name => "test")