Remove initializers no longer in Rails

These initializers either don't contain any code at all or contain code
that we never use. Since this was the case in most Rails applications,
they're no longer generated in new Rails applications [1].

So we're removing them as well.

[1] See pull requests 42538 and 43237 in https://github.com/rails/rails

config/initializers/application_controller_renderer.rb

@@ -1,8 +0,0 @@
-# Be sure to restart your server when you modify this file.
-
-# ActiveSupport::Reloader.to_prepare do
-#   ApplicationController.renderer.defaults.merge!(
-#     http_host: 'example.org',
-#     https: false
-#   )
-# end

config/initializers/backtrace_silencers.rb

@@ -1,8 +0,0 @@
-# Be sure to restart your server when you modify this file.
-
-# You can add backtrace silencers for libraries that you're using but don't wish to see in your backtraces.
-# Rails.backtrace_cleaner.add_silencer { |line| /my_noisy_library/.match?(line) }
-
-# You can also remove all the silencers if you're trying to debug a problem that might stem from framework code
-# by setting BACKTRACE=1 before calling your invocation, like "BACKTRACE=1 ./bin/rails runner 'MyClass.perform'".
-Rails.backtrace_cleaner.remove_silencers! if ENV["BACKTRACE"]

config/initializers/mime_types.rb

@@ -1,4 +0,0 @@
-# Be sure to restart your server when you modify this file.
-
-# Add new mime types for use in respond_to blocks:
-# Mime::Type.register "text/richtext", :rtf