Remove mention of generating CoffeeScript files in Preprocessing section

of the Asset Pipeline guide

This section previously declared that the Rails controller generator
generated an `app/assets/javascripts/projects.coffee` and an
`app/assets/stylesheets/projects.scss` file. Then, in
ab123a3, we removed the assertion that
`app/assets/javascripts/projects.coffee` would be generated (to reflect
the switch to webpacker for managing javascript in Rails 6). However,
the preceding sentence and the subsequent paragraph were still written
with the assumption that a CoffeeScript file would be generated.

Now we've reworded those parts to be consistent with the fact that Rails
no longer generates a CoffeeScript file.

[ci skip]

guides/source/asset_pipeline.md

@@ -510,17 +510,16 @@ might concatenate three CSS files together this way:
 ### Preprocessing
 
 The file extensions used on an asset determine what preprocessing is applied.
-When a controller or a scaffold is generated with the default Rails gemset, a
-CoffeeScript file and a SCSS file are generated in place of a regular JavaScript
-and CSS file. The example used before was a controller called "projects", which
-generated an `app/assets/stylesheets/projects.scss` file.
-
-In development mode, or if the asset pipeline is disabled, when these files are
-requested they are processed by the processors provided by the `coffee-script`
-and `sass` gems and then sent back to the browser as JavaScript and CSS
-respectively. When asset pipelining is enabled, these files are preprocessed and
-placed in the `public/assets` directory for serving by either the Rails app or
-web server.
+When a controller or a scaffold is generated with the default Rails gemset, an
+SCSS file is generated in place of a regular CSS file. The example used before
+was a controller called "projects", which generated an
+`app/assets/stylesheets/projects.scss` file.
+
+In development mode, or if the asset pipeline is disabled, when this file is
+requested it is processed by the processor provided by the `sass` gem and then
+sent back to the browser as CSS. When asset pipelining is enabled, this file is
+preprocessed and placed in the `public/assets` directory for serving by either
+the Rails app or web server.
 
 Additional layers of preprocessing can be requested by adding other extensions,
 where each extension is processed in a right-to-left manner. These should be