Source

File: wp-admin/includes/class-wp-filesystem-ftpext.php

public function delete( $file, $recursive = false, $type = false ) {
		if ( empty( $file ) ) {
			return false;
		}

		if ( 'f' === $type || $this->is_file( $file ) ) {
			return ftp_delete( $this->link, $file );
		}

		if ( ! $recursive ) {
			return ftp_rmdir( $this->link, $file );
		}

		$filelist = $this->dirlist( trailingslashit( $file ) );

		if ( ! empty( $filelist ) ) {
			foreach ( $filelist as $delete_file ) {
				$this->delete( trailingslashit( $file ) . $delete_file['name'], $recursive, $delete_file['type'] );
			}
		}

		return ftp_rmdir( $this->link, $file );
	}