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
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This is a partial backport of Core PR15632