Remove grahamc from the security team (#1143)

After a long tenure on many teams, and founding the security team -- it
is beyond time for me to step down and leave it in the very capable
hands of the other members.

I'm around if y'all need me :), and thanks for taking it to greater
heights!

community/teams/security.tt

@@ -38,13 +38,6 @@
     <h2>Members</h2>
 
     <ul>
-      <li>
-        <p>
-          <a href="https://discourse.nixos.org/u/grahamc">Graham Christensen</a>,
-          <a href="mailto:graham@determinate.systems">graham@determinate.systems</a>
-          (gchristensen)
-        </p>
-      </li>
       <li>
         <p>
           <a href="https://discourse.nixos.org/u/hexa">Martin Weinelt</a>,