[backport#15931 9/9] Remove getBlockDepth method from Chain::interface

Summary:
Pass conflicting height in CWallet::MarkConflicted

https://github.com/bitcoin/bitcoin/pull/15931/commits/36b68de5b2938722911db900ca299f7008780d01

Depends on D7513

Concludes backport of Core PR15931

Test Plan:

ninja check check-functional

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7514