Remove unnecessary TODO comment

We don't implement anonymous key exchanges, and we don't intend to, so it can
never happen that an unauthenticated server requests a certificate from us.
diff --git a/library/ssl_cli.c b/library/ssl_cli.c
index 1d22d15..5ce7d25 100644
--- a/library/ssl_cli.c
+++ b/library/ssl_cli.c
@@ -2581,9 +2581,6 @@
 
     ssl->record_read = 0;
 
-    // TODO: handshake_failure alert for an anonymous server to request
-    // client authentication
-
     /*
      *  struct {
      *      ClientCertificateType certificate_types<1..2^8-1>;