DEV: Remove ember-cli flags from the backend (#17147)

…and other auxiliary code

* Restore `QUNIT_EMBER_CLI` flag warning
* Add `ALLOW_EMBER_CLI_PROXY_BYPASS`

lib/ember_cli.rb

@@ -10,19 +10,6 @@ module EmberCli
     vendor.js
   )
 
-  ALIASES ||= {
-    "application" => "discourse",
-    "discourse/tests/test-support-rails" => "test-support",
-    "discourse/tests/test-helpers-rails" => "test-helpers"
-  }
-
-  def self.enabled?
-    if !Rails.env.production? && ENV["EMBER_CLI_PROD_ASSETS"] == "0"
-      STDERR.puts "The 'legacy' ember environment is discontinued. Running with ember-cli assets. Remove the EMBER_CLI_PROD_ASSETS=0 flag."
-    end
-    true
-  end
-
   def self.script_chunks
     return @@chunk_infos if defined? @@chunk_infos
 
@@ -40,17 +27,7 @@ module EmberCli
     {}
   end
 
-  # Some assets have changed name following the switch
-  # to ember-cli. When the switch is complete, we can
-  # drop this method and update all the references
-  # to use the new names
-  def self.transform_name(name)
-    return name if !enabled?
-    ALIASES[name] || name
-  end
-
   def self.is_ember_cli_asset?(name)
-    return false if !enabled?
     ASSETS.include?(name) || name.start_with?("chunk.")
   end
 end