Source

File: wp-admin/includes/class-wp-filesystem-ssh2.php

public function delete( $file, $recursive = false, $type = false ) {
		if ( 'f' === $type || $this->is_file( $file ) ) {
			return ssh2_sftp_unlink( $this->sftp_link, $file );
		}

		if ( ! $recursive ) {
			return ssh2_sftp_rmdir( $this->sftp_link, $file );
		}

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

		if ( is_array( $filelist ) ) {
			foreach ( $filelist as $filename => $fileinfo ) {
				$this->delete( $file . '/' . $filename, $recursive, $fileinfo['type'] );
			}
		}

		return ssh2_sftp_rmdir( $this->sftp_link, $file );
	}