Source

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

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

		if ( 'f' === $type || $this->is_file( $file ) ) {
			return $this->ftp->delete( $file );
		}

		if ( ! $recursive ) {
			return $this->ftp->rmdir( $file );
		}

		return $this->ftp->mdel( $file );
	}