Remove test_unit plugin_generator

While looking for templates using the `.rb` extension instead of
`.rb.tt`, my search ended up finding this `test_helper.rb` file.

```
$ fd | rg "/templates/.*\.rb$"
./actioncable/lib/rails/generators/channel/templates/application_cable/channel.rb
./actioncable/lib/rails/generators/channel/templates/application_cable/connection.rb
./railties/lib/rails/generators/test_unit/plugin/templates/test_helper.rb
```

Instead of correcting the extension, I opted to remove it because the
`test_unit:plugin` generator appears to be unused.

Co-Authored-By: Jean Boussier <[email protected]>

Author: skipkayhil

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

@@ -181,18 +181,18 @@ def test_no_color_sets_proper_shell
 
   def test_fallbacks_for_generators_on_find_by_namespace
     Rails::Generators.fallbacks[:remarkable] = :test_unit
-    klass = Rails::Generators.find_by_namespace(:plugin, :remarkable)
+    klass = Rails::Generators.find_by_namespace(:integration, :remarkable)
     assert klass
-    assert_equal "test_unit:plugin", klass.namespace
+    assert_equal "test_unit:integration", klass.namespace
   ensure
     Rails::Generators.fallbacks.delete(:remarkable)
   end
 
   def test_fallbacks_for_generators_on_find_by_namespace_with_context
     Rails::Generators.fallbacks[:remarkable] = :test_unit
-    klass = Rails::Generators.find_by_namespace(:remarkable, :rails, :plugin)
+    klass = Rails::Generators.find_by_namespace(:remarkable, :rails, :integration)
     assert klass
-    assert_equal "test_unit:plugin", klass.namespace
+    assert_equal "test_unit:integration", klass.namespace
   ensure
     Rails::Generators.fallbacks.delete(:remarkable)
   end