net: ppp: ipcp: Remove dead code

If nack_idx > 0, then the count_rej must be > 0. This means that
the "code" variable will never be set to PPP_CONFIGURE_NACK.

Fixes #22436
Coverity-CID: 207975

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>