Reinstate carbon balance check `err_found` for FATES cases

[glemieux]

It appears that an older commit which was avoiding ending FATES runs due to carbon imbalance was accidentally reintroduced with #3986 after it had been removed via #3990:

https://github.com/E3SM-Project/E3SM/blob/3fa659c8cb50895b982e1ed7857628195182cf8e/components/elm/src/biogeochem/EcosystemBalanceCheckMod.F90#L288-L317

The if (.not. use_fates) then statement should be removed. Note that the phosphorous and nitrogen checks that were introduced via #3990 as well appear to be intact.

cc @rgknox @bishtgautam