Remove unnecessary condition to skip tag list test

This file only has tests related to tags; if the model doesn't have
tags, we simply wouldn't include `it_behaves_as` in their tests instead
of including it and then skipping it.

spec/shared/models/sanitizable.rb

@@ -2,12 +2,6 @@
  let(:sanitizable) { build(model_name(described_class)) }
 
  describe "#tag_list" do
- before do
- unless described_class.included_modules.include?(Taggable)
- skip "#{described_class} does not have a tag list"
- end
- end
-
  it "sanitizes the tag list" do
  sanitizable.tag_list = "user_id=1"