Follow-up(add extension API): Remove ExtensionInterface and related functions/implementations

[joelanford]

Current plan is to remove ClusterExtension, so no need to introduce an abstraction that will very shortly obsolete when we do remove ClusterExtension. If plans change and ClusterExtension sticks around, we can re-assess the need for an abstraction.

[joelanford]

Once #642 merges, the types/functions mentioned in this issue become dead code. I've got a branch ready to rebase after that merges.