wallet2: fix on_reorg callback location

[j-berman]

Calls the on_reorg callback inside handle_reorg, instead of inside detach_blockchain.

detach_blockchain can be called even where there isn't a reorg (see how scan_tx uses detach_blockchain). handle_reorg should be called every time there is a reorg.

[selsta]

Is this relevant for release branch?

[j-berman]

The on_reorg callback hasn't been added on the release branch yet. Opened #9008 for release (didn't see a PR for it).