Remove incorrect KDC assertion

Remove incorrect KDC assertion

The assertion in return_enc_padata() is reachable because
kdc_make_s4u2self_rep() may have previously added encrypted padata.
It is no longer necessary because the code uses add_pa_data_element()
instead of allocating a new list.

CVE-2018-20217:

In MIT krb5 1.8 or later, an authenticated user who can obtain a TGT
using an older encryption type (DES, DES3, or RC4) can cause an
assertion failure in the KDC by sending an S4U2Self request.

[ghudson@mit.edu: rewrote commit message with CVE description]

ticket: 8767 (new)
tags: pullup
target_version: 1.17
target_version: 1.16-next
target_version: 1.15-next