This was actually redundant anyway since HIL itself applied a similar
rule where any partially-unknown list would be automatically flattened
to a single unknown value.
However, now we're changing HIL to explicitly permit partially-unknown
lists so that we can allow the index operator [...] to succeed when
applied to one of the elements that _is_ known.
This, in conjunction with hashicorp/hil#51 and hashicorp/hil#52,
fixes#3449.
Martin Atkins