Long term goal should be to provide alternative database connectors for our wallet. I have worked out a flexible adaptor like pattern that would allow a drop-in-alternative/replacement for BDB (https://github.com/jonasschnelli/bitcoin/tree/2016/08/bdb_abstract_prework).
This is a small, easy-to-review trivial PR towards this direction.
CDB::ReadAtCursor() has some unused flags handling (which would be relative hard to abstract for other databases). It seems like it only used to set a filter for the key (DB_SET_RANGE).
This removes unused code and simplifies CDB::ReadAtCursor() logic.