This page won't have a response on success or failure. You would have to check the webassist/security_assist/helpergroupsrulesphp.php file to see the correct rule name to use, but probably you would remove all of this from the bottom of the page:
<?php if(WA_Auth_RulePasses("Validated form")){ // Begin Show Region ?>
<p><span class="loginfail">Invalid username or password - please try again.</span></p>
<?php } // End Show Region ?>
<?php if(WA_Auth_RulePasses("Log in success")){ // Begin Show Region ?>
<p>You have been logged in</p>
<?php } // End Show Region ?>
<?php if(WA_Auth_RulePasses("Failed log in")){ // Begin Show Region ?>
<p><span class="loginfail">Invalid username or password - please try again.</span></p>
<?php } // End Show Region ?>
<?php if(WA_Auth_RulePasses("Emailed Password")){ // Begin Show Region ?>
<p>Password information emailed, please check your inbox</p>
<?php } // End Show Region ?>
<?php if(WA_Auth_RulePasses("Successful update")){ // Begin Show Region ?>
<p>Registration completed successfully, please log in to access the site</p>
<?php } // End Show Region ?>
and instead add something like:
<?php if(WA_Auth_RulePasses("Logged In To trade_users_app_temp")){ // Begin Show Region ?>Log in success<?php } // End Show Region ?>