Revert "WP_Icons_Registry: Add filter icons_registry_core_manifest"

We can think about this later on. I don't want to expose things
unnecessarily, but still wanted to show the concept.

This reverts commit 40dba1d.

Author: mcsf

src/wp-includes/class-wp-icons-registry.php

@@ -44,7 +44,6 @@ protected function __construct() {
 		$icons_directory = __DIR__ . '/icons/';
 		$icons_directory = trailingslashit( $icons_directory );
 		$manifest_path   = $icons_directory . 'manifest.php';
-		$manifest_path   = apply_filters( 'icons_registry_core_manifest', $manifest_path );
 
 		if ( ! is_readable( $manifest_path ) ) {
 			wp_trigger_error(