Plugin uninstall function is not working

register_deactivation_hook() is launched when a plugin is deactivated. For plugin uninstall, there is register_uninstall_hook().

WordPress plugins can also have a file called uninstall.php that is ran whenever the plugin is uninstalled. To prevent the uninstall.php file from being triggered directly, the state of a constant called WP_UNINSTALL_PLUGIN should be checked before launching the codes for uninstall.

Your uninstall.php could be:

<?php
//if uninstall not called from WordPress exit
if ( !defined( 'WP_UNINSTALL_PLUGIN' ) )
    exit ();

global $wpdb;       
$sortsearchtitle_db_version = '1.0';
$table = $wpdb->prefix."sortsearchresult";
$wpdb->query("DROP TABLE IF EXISTS $table");
delete_option('sortsearchtitle_db_version');