Fix transaction payment runtime api (#6792)

The transaction payment runtime api used its own extrinsic generic
parameter. This is wrong, because this resulted in using always the
native extrinsic. If there was a runtime upgrade that changed the
extrinsic in some way, it would result in the api breaking. The correct
way is to use the `Extrinsic` from the `Block` parameter. This is on the
node side the opaque extrinsic and on the runtime side the real extrinsic.