Clean up references to answer distribution-like things

In the near future an answer distribution report will be displayed in the
data download reports table.  In order to avoid confusion, we need to clean
up several misleading labels and remove a redundant feature.

Note that the redundant feature is surrounded by a feature flag so that it
can be disabled once the report goes live in the table.

Fixes: AN-694
Reviewers: @sarina, @rocha, @lamagnifica
FYI: @stroilova, @brianhw