docs: remove link to MissingFunctionality wiki page

Outdated, features.txt is used instead.

Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>

docs/helpwanted.html

@@ -49,8 +49,6 @@ You can find some further To-do lists here:
 <ul>
   <li><a href="https://cgit.freedesktop.org/mesa/mesa/tree/docs/features.txt">
     <b>features.txt</b></a> - Status of OpenGL 3.x / 4.x features in Mesa.</li>
-  <li><a href="https://dri.freedesktop.org/wiki/MissingFunctionality">
-    <b>MissingFunctionality</b></a> - Detailed information about missing OpenGL features.</li>
 </ul>
 
 <p>