Do not generate configuration for sprockets if propshaft is used

[javierav]

Motivation / Background

This Pull Request has been created because currently, when a new rails project is initialized using propshaft, configuration for sprockets is generated in the file config/initializers/assets.rb.

# Precompile additional assets.
# application.js, application.css, and all non-JS/CSS in the app/assets
# folder are already added.
# Rails.application.config.assets.precompile += %w( admin.js admin.css )

Although this part of the code is generated with comments, it can mislead especially people who are less experts in the framework.

Detail

This Pull Request includes the necessary changes so that that part is not generated in the configuration file.