wallet: Remove Verify and IsLoaded methods

Summary:
Checks are now consolidated in MakeBerkeleyDatabase function instead of
happening in higher level code.

This commit does not change behavior except for error messages which now
include more complete information.

This is a backport of core#19619 [4/8]
https://github.com/bitcoin/bitcoin/pull/19619/commits/3c815cfe54087fd139169161d2fd175e99840e6a

Depends on D10226

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10227