walletdb: Remove unused static functions from walletdb.h

Summary:
VerifyEnvironment and VerifyDatabaseFile were removed in D8617/PR19324, but their declarations weren't. Remove those.

This is a backport of core#19457 [3/3]
https://github.com/bitcoin/bitcoin/pull/19457/commits/0e279fe4899beae8630264ef1fe420dd71f29d5d

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D9079