Stop passing current height to `PackageTemplate::build_package`

Stop passing current height to `PackageTemplate::build_package`

Now that we don't store the confirmation height of the inputs
being spent, passing the current height to
`PackageTemplate::build_package` is useless - we only use it to set
the height at which we should next bump the fee, but we just want
it to be "next block", so we might as well use `0` and avoid the
extra argument. Further, in one case we were already passing `0`,
so passing the argument is just confusing as we can't rely on it
being set.

Note that this does remove an assertion that we never merge
packages that were crated at different heights, and in the future
we may wish to do that (as there's no specific reason not to), but
we do not currently change the behavior.