Remove tests checking that the theme is present in the manifest

I’ve removed that portion of the comment since changing the theme doesn’t affect the content stored offline anymore.

test/app_test.rb

@@ -100,19 +100,6 @@ class AppTest < MiniTest::Spec
       refute_includes last_response.body, 'foo'
     end
 
-    it "has the word 'default' when no 'dark' cookie is set" do
-      get '/manifest.appcache'
-      assert_includes last_response.body, '# default'
-      refute_includes last_response.body, '# dark'
-    end
-
-    it "has the word 'dark' when the cookie is set" do
-      set_cookie('dark=1')
-      get '/manifest.appcache'
-      assert_includes last_response.body, '# dark'
-      refute_includes last_response.body, '# default'
-    end
-
     it "sets default size" do
       get '/manifest.appcache'
       assert_includes last_response.body, '20rem'