Remove `modified` enum from customer because its not supported.

Customers are based on `users` table which does not have a modified or similar column to track last modified timestamp, so this param cannot be supported at this time.

src/Controllers/Version3/class-wc-rest-customers-controller.php

@@ -304,16 +304,4 @@ class WC_REST_Customers_Controller extends WC_REST_Customers_V2_Controller {
 
 		return $this->add_additional_fields_schema( $schema );
 	}
-
-    /**
-	 * Add new options for 'orderby' to the collection params.
-	 *
-	 * @return array
-	 */
-	public function get_collection_params() {
-    		$params                    = parent::get_collection_params();
-    		$params['orderby']['enum'] = array_merge( $params['orderby']['enum'], array( 'modified' ) );
-
-    		return $params;
-    	}
 }