krb5_encode: remove unused argument

Coverity CID 1241957. Removed the unused argument. As this struct and
pointer now are used only for krb5, there's no need to keep unused
function arguments around.

lib/security.c

@@ -295,7 +295,7 @@ static void do_sec_send(struct connectdata *conn, curl_socket_t fd,
       prot_level = conn->command_prot;
   }
   bytes = conn->mech->encode(conn->app_data, from, length, prot_level,
-                             (void**)&buffer, conn);
+                             (void**)&buffer);
   if(!buffer || bytes <= 0)
     return; /* error */