Revert "Accept binaries as struct keys."

This reverts commit 11115054b9f379c91e2bb998533dc7abd806be3b.

This has been rejected upstream on the grounds that

Received = <<"{\"definitelynotanatom\":\"B\"}">>,
Parsed = erlson:from_json(Received),
Improved = [{foo, <<"bar">>} | Parsed],
Result = erlson:from_nested_proplist(Added).

can be better represented as

Received = <<"{\"definitelynotanatom\":\"B\"}">>,
Parsed = erlson:from_json(Received),
Result = Parsed#{foo = <<"bar">>}.

The change would also entail an API change for erlson, which upstream
was not happy with.

This is a reasonable decision -- so reverting here and updating the code that
was calling this as above.