Clean up client version handling

[cornelius]

The Bitcoin Core codebase has a couple of places where there are checks on the client version to enable features which where introduced in certain releases. As we are starting fresh in Unit-e, we have all these features from the beginning so there is no need for these conditionals right now. We will need this at some point in the future, though, when we introduce features which need to be backwards compatible in some way. This depends on the nature of the features.

For now we have only done a minimal change to introduce the unit-e version number (#945, #969). We should revisit this question and evaluate how we can do it in a cleaner way which suits the needs of unit-e while still allowing us to sync with upstream.