auth: remove unused AuthTicket::renew_after member variable

auth: remove unused AuthTicket::renew_after member variable

The AuthTicket::renew_after field is only set in init_timestamps() and
read by dump() for debugging purposes. It has no functional use cases
and causes encoding/decoding inconsistencies.

During decoding, this field remains unchanged, creating discrepancies
between original and decoded values. This issue was masked because
check-generated.sh and readable.sh reused struct instances, preserving
stale field values across encode/decode cycles.

An upcoming change will allocate fresh instances for each decode
operation, which would expose these inconsistent values.

Remove the unused field to eliminate the encoding inconsistency and
simplify the codebase.

Signed-off-by: Kefu Chai <tchaikov@gmail.com>