CaptchasDotNet: don't make the captcha image a link to captcha.net

This is horrible user experience on mobile, user can accidentally touch the
image and then get redirected to captcha.net website which is generally not
useful. As far as a I understand their conditions of use, they don't require
use to make a link to their website.

product/ERP5Form/CaptchasDotNet.py

@@ -85,10 +85,9 @@ class CaptchasDotNet:
 
     def image (self, random, id = 'captchas.net'):
         return '''
-        <a href="http://captchas.net"><img
-            class="captchas_dot_net"
+        <img class="captchas_dot_net"
             id="%(id)s" src="%(source)s" width="%(width)d" height="%(height)d"
-            alt="The CAPTCHA image" /></a>
+            alt="The CAPTCHA image" />
         <script type="text/javascript">
           <!--
           function captchas_image_error (image)