Remove documentation of deleted function

It's even removing a BUGS entry!
Reviewed-by: NEmilia Käsper <emilia@openssl.org>
Reviewed-by: NRich Salz <rsalz@openssl.org>
Reviewed-by: NRichard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2797)

doc/man3/DES_random_key.pod

@@ -254,8 +254,6 @@ bytes.
 
 =head1 BUGS
 
-DES_3cbc_encrypt() is flawed and must not be used in applications.
-
 DES_cbc_encrypt() does not modify B<ivec>; use DES_ncbc_encrypt()
 instead.