Fix transaction payment runtime api

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.