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