Make flush_gdd20 automatic

> In 827632e12, I resolved the "two `flush_gdd20`s" question by removing the one that was a member of `TemperatureType`. Having that alongside `flush_gdd20` from `clm_varctl` meant that they could disagree when restarting, and it's not obvious which should win.
>
> Note that the user being able to request a flush at all is dangerous. If they start a run with it true, they might continue that run without setting it to false. Instead, the restart decision should be made by the model—if a gridcell has different prescribed GDD20 season from what its restart file says, then that gridcell should be flushed.
>
> I will plan to make that change in a future update, post-code-freeze. It only affects things when streaming GDD20 seasons, which will not be a default behavior, so I think it's okay to leave things as they are now.

_Originally posted by @samsrabin in https://github.com/ESCOMP/CTSM/issues/2593#issuecomment-2249100289_