Revert "Use sass compressor as the default CSS compressor"

This reverts commit 3eef1c7.
The css compressor should be set in sass-rails plugin. Check
rails/sass-rails@339529f
for the fix.

railties/lib/rails/generators/rails/app/templates/config/environments/production.rb.tt

@@ -25,7 +25,7 @@
   <%- unless options.skip_sprockets? -%>
   # Compress JavaScripts and CSS.
   config.assets.js_compressor  = :uglifier
-  config.assets.css_compressor = :sass
+  # config.assets.css_compressor = :sass
 
   # Whether to fallback to assets pipeline if a precompiled asset is missed.
   config.assets.compile = false