posts: show only last successful flag on deleted posts.

In the "This post was deleted" notice, show only the last successful
flag, instead of all previous flags (which may include rejected flags).

test/functional/posts_controller_test.rb

@@ -540,6 +540,15 @@ class PostsControllerTest < ActionDispatch::IntegrationTest
         end
       end
 
+      context "a deleted post" do
+        should "render" do
+          @post.delete!("no", user: @user)
+          get post_path(@post)
+
+          assert_response :success
+        end
+      end
+
       context "with pools" do
         should "render the pool list" do
           as(@user) { @post.update(tag_string: "newpool:comic") }