tx_builder does not handle decred consistently

[prestwich]

Sometimes it uses make_decred, sometimes it checks if the network is set to decred.

It should uniformly check if the network is set to decred.

[prestwich]

I can't remember if I left the make_decred pattern on purpose as a convenience

[drmoog]

make_decred_input() is only called by the unit tests, so I suspect so. make_decred_witness is called within make_witness_input_and_witness() however, after checking network is set to decred.