Drop autoprefixer-rails from dependencies

No need to force it on the whole application asset pipeline in 2023,
and it's easy to add it back locally if really needed.

backend/lib/spree/backend.rb

@@ -7,7 +7,6 @@
 require 'sassc-rails'
 require 'handlebars_assets'
 require 'font-awesome-rails'
-require 'autoprefixer-rails'
 require 'jbuilder'
 require 'kaminari'
 require 'responders'

backend/solidus_backend.gemspec

@@ -33,6 +33,5 @@ Gem::Specification.new do |s|
   s.add_dependency 'responders'
   s.add_dependency 'sassc-rails'
 
-  s.add_dependency 'autoprefixer-rails', '~> 10.2', '>= 10.2.5.1'
   s.add_dependency 'handlebars_assets', '~> 0.23'
 end