Delete the initializer `initialize_dependency_mechanism`

The concept of dependencies mechanism disappears.

We do not remove the attribute in AS::Dependencies itself here, that
class is going to have a lot of code delete later.

guides/source/configuring.md

@@ -274,18 +274,18 @@ Every Rails application comes with a standard set of middleware which it uses in
     Rails.application.config.hosts << ".product.com"
     ```
 
-    You can exclude certain requests from Host Authorization checks by setting 
+    You can exclude certain requests from Host Authorization checks by setting
     `config.host_configuration.exclude`:
 
     ```ruby
     # Exclude requests for the /healthcheck/ path from host checking
-    Rails.application.config.host_configuration = { 
-      exclude: ->(request) { request.path =~ /healthcheck/ } 
+    Rails.application.config.host_configuration = {
+      exclude: ->(request) { request.path =~ /healthcheck/ }
     }
-    ``` 
+    ```
 
-    When a request comes to an unauthorized host, a default Rack application 
-    will run and respond with `403 Forbidden`. This can be customized by setting 
+    When a request comes to an unauthorized host, a default Rack application
+    will run and respond with `403 Forbidden`. This can be customized by setting
     `config.host_configuration.response_app`. For example:
 
     ```ruby
@@ -1665,8 +1665,6 @@ Below is a comprehensive list of all the initializers found in Rails in the orde
 
 * `set_clear_dependencies_hook`: This initializer - which runs only if `cache_classes` is set to `false` - uses `ActionDispatch::Callbacks.after` to remove the constants which have been referenced during the request from the object space so that they will be reloaded during the following request.
 
-* `initialize_dependency_mechanism`: If `config.cache_classes` is true, configures `ActiveSupport::Dependencies.mechanism` to `require` dependencies rather than `load` them.
-
 * `bootstrap_hook`: Runs all configured `before_initialize` blocks.
 
 * `i18n.callbacks`: In the development environment, sets up a `to_prepare` callback which will call `I18n.reload!` if any of the locales have changed since the last request. In production this callback will only run on the first request.

railties/lib/rails/application/bootstrap.rb

@@ -64,11 +64,6 @@ module Rails
         end
       end
 
-      # Sets the dependency loading mechanism.
-      initializer :initialize_dependency_mechanism, group: :all do
-        ActiveSupport::Dependencies.mechanism = config.cache_classes ? :require : :load
-      end
-
       initializer :bootstrap_hook, group: :all do |app|
         ActiveSupport.run_load_hooks(:before_initialize, app)
       end

railties/test/application/configuration_test.rb

@@ -460,17 +460,6 @@ module ApplicationTests
       end
     end
 
-    test "initialize an eager loaded, cache classes app" do
-      add_to_config <<-RUBY
-        config.eager_load = true
-        config.cache_classes = true
-      RUBY
-
-      app "development"
-
-      assert_equal :require, ActiveSupport::Dependencies.mechanism
-    end
-
     test "application is always added to eager_load namespaces" do
       app "development"
       assert_includes Rails.application.config.eager_load_namespaces, AppTemplate::Application