This allows us to reference assumeutxo configuration by blockhash as
well as height; this is helpful in future changes when we want to
reference assumeutxo configurations before the block index is loaded.
Also remove the unused assumeutxo checkpoint for block 210.
This is a partial backport of core#27596
https://github.com/bitcoin/bitcoin/pull/27596/commits/434495a8c1496ca23fe35b84499f3daf668d76b8
Depends on D17633