wallet: rm get_onchain_request_status; just use is_onchain_invoice_paid
wallet: rm get_onchain_request_status; just use is_onchain_invoice_paid

There is some duplication between `wallet.get_onchain_request_status` and `wallet.is_onchain_invoice_paid`
(both in terms of code, and conceptually). `get_onchain_request_status` is used for incoming invoices (receive requests),
and `is_onchain_invoice_paid` is used for outgoing invoices. I think `get_onchain_request_status` existed first,
but as it uses txi/txo, it only works for ismine addresses, so `is_onchain_invoice_paid` was added later
(along with a `get_prevouts_by_scripthash` and corresponding new persisted data structure) to handle the non-ismine
addresses corresponding to outgoing invoices.

I think we could just merge the two functions together... (?)
and this PR does that.

spesmilo/electrum,pooler/electrum-ltc,spesmilo/electrum,pooler/electrum-ltc,pooler/electrum-ltc,pooler/electrum-ltc,spesmilo/electrum,spesmilo/electrum