building plugin and it is displaying above plugins page

You’re using add_menu_page wrong. The function bittech_login_settings should be only:

function bittech_login_settings() {
    add_menu_page(
        'BitTech Login Settings', 
        'BitTech Login Settings', 
        'add_users', 
        'bittech_settings', 
        'bittech_login_settings_callback'
    );
}

function bittech_login_settings_callback() {
    // content of the menu
}

And put the contents of the page in the callback function bittech_login_settings_callback. Also, these lines don’t make much sense:

$filename = "../wp-content/plugins/bittech_login/include/config.php";
$contents = file_get_contents($filename);
if($_POST) file_put_contents($filename, $_POST['field']);

You should probably store that field in the database with update_option and retrieve it with get_option().