I18N: Remove the requirement to call load_plugin_textdomain() / load_theme_textdomain().

By initially scanning the `wp-content/languages` directory and loading available MO files just-in-time, plugins and themes do not need to manually load text domains anymore.

Props swissspidy, ocean90.
Fixes #34114

git-svn-id: https://develop.svn.wordpress.org/trunk@37415 602fd350-edb4-49c9-b593-d223f7449a82

tests/phpunit/data/plugins/internationalized-plugin.php

@@ -0,0 +1,12 @@
+<?php
+/*
+Plugin Name: Dummy Plugin
+Plugin URI: https://wordpress.org/
+Description: For testing purposes only.
+Version: 1.0.0
+Text Domain: internationalized-plugin
+*/
+
+function i18n_plugin_test() {
+	return __( 'This is a dummy plugin', 'internationalized-plugin' );
+}