remove unused nChainSize
Summary: This stat is not used anywhere in the codebase. The getblockchaininfo already has a size_on_disk value that is arguably more useful for users.
Test Plan: ninja all check-all bitcoin-fuzzers
Reviewers: #bitcoin_abc, Fabien
Reviewed By: #bitcoin_abc, Fabien
Differential Revision: https://reviews.bitcoinabc.org/D17878