[wallet] Remove duplicate mapWallet lookups

Summary:
Backport of Core PR12276
Completes T518

Test Plan:

ninja check
bitcoin-qt

Wallet loads as expected.

Reviewers: deadalnix, Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2319