How do I remove g-recaptcha-response in an online form?

Check g-recaptcha-response

outside of the loop, there is no need to check it every time you iterate.

Then check that the variable key is $_POST

not g-recaptcha-response

before adding it to the message.

$mailTo = "$email_address";

$mailSubject = "$email_subject";

$mailBody = "The form values entered by the user are as follows: \n\n";

if(isset($_POST['g-recaptcha-response'])){$captcha=$_POST['g-recaptcha-response'];
}

foreach($HTTP_POST_VARS as $key=>$value)
{
    if(!$captcha)
    {
        echo "<meta HTTP-EQUIV=\"REFRESH\" content=\"0; url=$redirect_to_failed\">";
        exit;
    }
    $response=file_get_contents("https://www.google.com/recaptcha/api/siteverify?secret=&response=".$captcha."&remoteip=".$_SERVER['REMOTE_ADDR']);
    if($response.success==false)
    {
              echo '<h2>You are spammer! /h2>';
    }else
    {
              echo '<h2>Thanks for contacting us.</h2>';
    }
    if ($key != 'g-recaptcha-response') {
        $mailBody .= "$key = $value\n";
    }
}