wallet: remove db mode string

Summary:
We never need to open database in read-only mode as it's controlled
separately for every batch.

Also we can safely create database if it doesn't exist already
because require_existing option is verified in MakeDatabase
before creating a new WalletDatabase instance.

This is a backport of core#20130

Notes:

• walletdb_tests.cpp is an addtional ABC test suite introduced in D745
• ABC does not have a CWallet::MarkReplaced method

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10468