[backport#15680] [wallet] Remove ResendWalletTransactionsBefore

Summary:
This is only called from ResendWalletTransactions(), so bring it inline.

https://github.com/bitcoin/bitcoin/pull/15680/commits/ea1a2d8794ffb6b814e7120fb8ba0cc4aa7ebf3d

Depends on D5951

This completes the backport of Core PR15680

Test Plan:

ninja check check-functional

Reviewers: #bitcoin_abc, jasonbcox, deadalnix

Reviewed By: #bitcoin_abc, jasonbcox, deadalnix

Subscribers: jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D5952