Remove the legacy_connection_handling setter

This code is old and don't need to exist anymore.

Author: rafaelfranca

activerecord/lib/active_record.rb

@@ -268,14 +268,6 @@ def self.db_warnings_action=(action)
   singleton_class.attr_accessor :reading_role
   self.reading_role = :reading
 
-  def self.legacy_connection_handling=(_)
-    raise ArgumentError, <<~MSG.squish
-      The `legacy_connection_handling` setter was deprecated in 7.0 and removed in 7.1,
-      but is still defined in your configuration. Please remove this call as it no longer
-      has any effect."
-    MSG
-  end
-
   ##
   # :singleton-method: async_query_executor
   # Sets the async_query_executor for an application. By default the thread pool executor

railties/test/application/configuration_test.rb

@@ -4422,17 +4422,6 @@ def new(app); self; end
       ActiveRecord::Base.configurations = original_configurations
     end
 
-    test "raises an error if legacy_connection_handling is set" do
-      build_app(initializers: true)
-      add_to_env_config "production", "config.active_record.legacy_connection_handling = true"
-
-      error = assert_raise(ArgumentError) do
-        app "production"
-      end
-
-      assert_match(/The `legacy_connection_handling` setter was deprecated in 7.0 and removed in 7.1, but is still defined in your configuration. Please remove this call as it no longer has any effect./, error.message)
-    end
-
     test "raise_on_missing_translations = true" do
       add_to_config "config.i18n.raise_on_missing_translations = true"
       app "development"