This field was never user-configurable. We always overwrote the value with 120s from
NonUserSource. However, we also never copied the value from RuntimeConfig to consul.Config,
So the value in NonUserSource was always ignored, and we used the default value of 30s
set by consul.DefaultConfig.
All of this code is an unnecessary distraction because a user can not actually configure
this value.
This commit removes the fields and uses a constant value instad. Someone attempting to set
acl.disabled_ttl in their config will now get an error about an unknown field, but previously
the value was completely ignored, so the new behaviour seems more correct.
We have to keep this field in the AutoConfig response for backwards compatibility, but the value
will be ignored by the client, so it doesn't really matter what value we set.
