Barbican has recently bumped max_allowed_secret_in_bytes from 10 KB to
20 KB since the original value was too small for some certificates [1].
Remove custom value from the barbican.conf template, which anyway was
the same as the default configuration before the recent upstream change.

The upstream change was backported to Wallaby and has been proposed to
Victoria, Ussuri and Train [2], so this change should be backported too.

[1] https://review.opendev.org/c/openstack/barbican/+/783381
[2] https://review.opendev.org/q/I59d11c5c9c32128ab9d71eaecdf46dd2d789a8d1

Change-Id: I83e4cb48192c8024650a8d347363f6babb75ad90
Closes-Bug: #1957795