[backport#15931 8/9] Remove locked_chain from GetDepthInMainChain and its callers

Summary:
We don't remove yet Chain locks as we need to preserve lock
order with CWallet one until swapping at once to avoid
deadlock failures (spotted by --enable-debug)

https://github.com/bitcoin/bitcoin/pull/15931/commits/b66c429c56c85fa16c309be0b2bca9c25fdd3e1a

Depends on D7512

Partial backport of PR15931

Test Plan:

ninja check check-functional

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7513