Remove txindex migration code

Summary:

Add missing includes and forward declarations, remove unused ones

doc: Fix validation typo

Remove txindex migration code

PR description:

No supported version of Bitcoin used the legacy txindex, so all relevant nodes can be assumed to have upgraded. Thus, there is no need to keep this code any longer.

As a temporary courtesy, provide a one-time warning on how to free the disk space used by the legacy txindex.

This is a backport of core#22626
Depends on D12242

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D12244