util: Simplify path argument for CBlockTreeDB ctor
Summary:
This PR:
- simplifies path argument (datadir/blocks/index) for CBlockTreeDB constructor
- does not change behavior as GetBlocksDir() with unset "-blocksdir" returns the same path
- improves code readability
This is a backport of Core PR17059
Test Plan: ninja all check-all
Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix
Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix
Differential Revision: https://reviews.bitcoinabc.org/D8119