bugfix: Remove dangling wallet env instance and Delete walletView in WalletFrame::removeWallet

Summary:
Partial backport of Core PR13111
https://github.com/bitcoin/bitcoin/pull/13111/commits/0b82bac76d0f842bd2294a290388536951fbc576 (only the remaining change to wallet/db.cpp)
https://github.com/bitcoin/bitcoin/pull/13111/commits/fe65bdec237776dbe094339509dfd2e63329a832

Depends on D4254

Test Plan:

make check
test_runner.py

Reviewers: deadalnix, Fabien, jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D4255