Remove `ComputeSighash`

[danielabrozzoni]

Needs #1023

Use the rust-bitcoin methods instead, and contribute upstream if they're lacking. See #1023 (comment)

[notmandatory]

A likely small change but I think we should push to post 1.0 milestone.

[nondiremanuel]

Needs some more research to understand if it's needed for 1.0

[ValuedMammal]

I'm willing to have a go at this.