Merge #11870: wallet: Remove unnecessary mempool lock in ReacceptWalletTransactions

Summary:
5b25293 wallet: Remove unnecessary mempool lock in ReacceptWalletTransactions (João Barbosa)

Pull request description:

Tree-SHA512: 13b922c6c9b5ca95a77742050f449366b80bdd7819c34e7ca09af8a4bd68085f4d0c6e0cde119c403f661499f97f2c465071a8047a7d794268e8d2dfe909e6d5

Backport Core PR11870
https://github.com/bitcoin/bitcoin/pull/11870/files

Test Plan:

make check
test_runner.py

Reviewers: #bitcoin_abc, deadalnix, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D3469