Remove the ENV flag, yagni.

actionpack/lib/sprockets/assets.rake

@@ -1,9 +1,6 @@
 namespace :assets do
   desc "Compile all the assets named in config.assets.precompile"
   task :precompile do
-    # let rails know we're precompiling assets
-    ENV["RAILS_ASSETS_PRECOMPILE"] = 'true'
-
     # We need to do this dance because RAILS_GROUPS is used
     # too early in the boot process and changing here is already too late.
     if ENV["RAILS_GROUPS"].to_s.empty? || ENV["RAILS_ENV"].to_s.empty?

railties/test/application/assets_test.rb

@@ -106,16 +106,6 @@ def precompile!
       assert !File.exists?("#{app_path}/public/assets/something.min.css")
     end
 
-    test "precompile sets flag notifying rails its precompiling" do
-      add_to_config = <<-RUBY
-        initializer :check_assets_precompile, :group => :assets do
-          raise "ENV RAILS_ASSETS_PRECOMPILE not set" unless ENV["RAILS_ASSETS_PRECOMPILE"]
-        end
-      RUBY
-      precompile!
-      assert $?.success?
-    end
-
     test "asset pipeline should use a Sprockets::Index when config.assets.digest is true" do
       add_to_config "config.assets.digest = true"
       add_to_config "config.action_controller.perform_caching = false"