Remove unused properties and align docs with abstract-level

Removes the following:

- bufferKeys (replaced by encodings.buffer)
- idempotentOpen (was temporary to differentiate with levelup)
- passiveOpen (was temporary to differentiate with levelup)
- serialize (abstract-leveldown feature)
- openCallback (levelup feature).

Most of these were added as a temporary mechanism to run test suites
against each other, until those were merged into `abstract-level`.

Author: vweevers