[wallet] Move methods from Chain::Lock interface to simple Chain

Summary:
Remove findPruned and findFork, no more used after 17954.

Backport of Core PR16426 part [4/5] : https://github.com/bitcoin/bitcoin/pull/16426/commits/841178820d31e1c24a00cb2c8fc0b1fd2f126f56

Depends on D7936

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, majcosta, Fabien

Reviewed By: #bitcoin_abc, majcosta, Fabien

Subscribers: majcosta

Differential Revision: https://reviews.bitcoinabc.org/D7937