Last release we added a type alias for `Psbt`, now lets just rename the
type and be done with it.
Includes re-export at the crate root because `bitcoin::Psbt` is clear
and obvious.
AFAICT we literally never used this; it was available only on the
bitcoin targets and not the honggfuzz ones; AFL has a broken dep
tree (or at least, requires some more MSRV pins that I did not care
to investigate).
Just remove it entirely.