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.
The existing wallet_resendwallettransactions.py test only tests the
resendwallettransactions RPC. It does not test whether transactions are
actually rebroadcast, or whether the rebroadcast logic is called on a
timer.
This commit updates the test to not use the resendwallettransactions RPC and
test that transactions are rebroadcast on a timer.