[backport#15632] [wallet] Schedule tx rebroadcasts in wallet

Summary:
Removes the now-unused Broadcast/ResendWalletTransactions interface from
validationinterface.

The wallet_resendwallettransactions.py needs a sleep added at the start
to make sure that the rebroadcast scheduler is warmed up before the next
block is mined.

https://github.com/bitcoin/bitcoin/pull/15632/commits/52b760fc6a9b26e405a0553ee8285b0f03ca1604

This is a partial backport of Core PR15632

Test Plan:

cmake .. -GNinja -DENABLE_WERROR=ON
ninja check-all

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6031