Remove post changes report.

This was a mod-only report that used Google BigQuery to search post
versions by tag. 2b4ee0ee8 allows all users to search post versions by
tag, so this report is no longer necessary.

test/functional/reports_controller_test.rb

@@ -17,13 +17,6 @@ class ReportsControllerTest < ActionDispatch::IntegrationTest
       end
     end
 
-    context "post_versions action" do
-      should "render" do
-        get_auth reports_post_versions_path, @mod
-        assert_response :success
-      end
-    end
-
     context "upload_tags action" do
       should "render" do
         get reports_upload_tags_path(user_id: @users.first)