This just drops three `interfaces::Chain` methods replacing them with other calls.
Motivation for removing these chain methods:
- Need to get rid of findFirstBlockWithTimeAndHeight for [[#10102 | https://github.com/bitcoin/bitcoin/pull/10102]], which doesn't
support overloaded methods
- Followup from
https://github.com/bitcoin/bitcoin/pull/16426#discussion_r412487403
- phantomcircuit comments about findNextBlock test
http://www.erisian.com.au/bitcoin-core-dev/log-2020-06-06.html#l-214
Behavior is not changing in any way here. A TODO comment in
ScanForWalletTransactions was removed, but just because it was invalid (see
https://github.com/bitcoin/bitcoin/pull/19195#discussion_r448020762), not
because it was implemented.
This is a backport of core#19425 [1/3]
https://github.com/bitcoin/bitcoin/pull/19425/commits/3fbbb9a6403a86fbed3d5d9f7939998922593377