posts: fix expunging posts not removing image from iqdb.

`File.exists?(preview_file_path)` is always false because the file has
already been deleted by this point. Remove the check entirely.

app/models/post.rb

@@ -1713,9 +1713,7 @@ class Post < ActiveRecord::Base
     end
 
     def remove_iqdb_async
-      if File.exists?(preview_file_path) && Post.iqdb_enabled?
-        Post.iqdb_sqs_service.send_message("remove\n#{id}")
-      end
+      Post.remove_iqdb(id)
     end
 
     def update_iqdb

test/helpers/iqdb_test_helper.rb

@@ -16,6 +16,7 @@ module IqdbTestHelper
 
     service = mock_sqs_service.new
     Post.stubs(:iqdb_sqs_service).returns(service)
+    Post.stubs(:iqdb_enabled?).returns(true)
 
     Danbooru.config.stubs(:iqdbs_auth_key).returns("hunter2")
     Danbooru.config.stubs(:iqdbs_server).returns("http://localhost:3004")

test/unit/post_test.rb

@@ -52,6 +52,15 @@ class PostTest < ActiveSupport::TestCase
         assert_equal(false, File.exists?(@post.file_path))
       end
 
+      should "remove the post from iqdb" do
+        mock_iqdb_service!
+        Post.iqdb_sqs_service.expects(:send_message).with("remove\n#{@post.id}")
+
+        TestAfterCommit.with_commits(true) do
+          @post.expunge!
+        end
+      end
+
       context "that is status locked" do
         setup do
           @post.update_attributes({:is_status_locked => true}, :as => :admin)