start and end term can be integers in 2i record

Pair-programmed with @reiddraper

Since 2i queries can be over binaries and integers, the record should
reflect this.
Remove dead code

Pair-programmed with @reiddraper

Since this function is only ever called from one place, and that place
does error checking, this pattern will never match. Dialyzer told me so.
Merge pull request #907 from basho/feature/riak_kv_pb_index-dialyzer

Feature/riak kv pb index dialyzer

Reviewed-by: reiddraper