Removed unnecessary signTxOutputCustomReader.

[`signTxOutputCustomReader`](https://github.com/conformal/btcscript/blob/master/script.go#L1126) has an unused parameter `reader io.Reader` and [`signTxOutput`](https://github.com/conformal/btcscript/blob/master/script.go#L1119) delegats to this function. This patch removes the unnecessary `signTxOutputCustomReader` and just makes `signTxOutput` perform the work. Maybe `signTxOutputCustomReader` is a leftover from a previous implementation?