Remove unnecessary `doc` directory deletion

Since 553b6958, `doc` directory is not created in application.

railties/lib/rails/generators/rails/plugin/plugin_generator.rb

@@ -120,7 +120,6 @@ def test_dummy_assets
     def test_dummy_clean
       inside dummy_path do
         remove_file "db/seeds.rb"
-        remove_file "doc"
         remove_file "Gemfile"
         remove_file "lib/tasks"
         remove_file "public/robots.txt"

railties/test/generators/plugin_generator_test.rb

@@ -491,7 +491,6 @@ def test_unnecessary_files_are_not_generated_in_dummy_application
     assert_no_file "test/dummy/public/robots.txt"
     assert_no_file "test/dummy/README.md"
     assert_no_directory "test/dummy/lib/tasks"
-    assert_no_directory "test/dummy/doc"
     assert_no_directory "test/dummy/test"
     assert_no_directory "test/dummy/vendor"
     assert_no_directory "test/dummy/.git"