Remove unnecessary shipping rates callback

[mamhoff]

The callback Spree::Order#ensure_available_shipping_rates was
explicitly written to be run before delivery. Running it before the
complete step leaves the order in the confirm state, but without
any shipments, and results in an error message does not make sense
in that context ("We were unable to calculate shipping rates"). In fact,
we have not even tried.

It's almost impossible to setup an order such that this callback would
actually trigger in a standard Solidus installation.

The commit that introduced the callback, 7ba53b2, intends to move line
item availability validations to the before complete callback, instead
of "all the time". That makes sense, but this callback has somehow sneaked
in there without any explanation of why its necessary.

I delete a model spec that was introduced in e7450ec, testing the behaviour
in question. The spec set the order up in a way that a normal checkout never would.
If you get through delivery and payment to the confirm step, and then
do literally anything to the order, it will be reset to either cart or address
state.

I edit another spec to test what the callback should actually do: Alert the user
that for their recently entered shipping address, no shipping rates can be calculated.

[tvdeyen]

Code and behavior wise this is more correct now, but I don't know any of the rationale behind the former change.

[mamhoff]

I think this accidentally slipped into the codebase, actually.

[jhawthorn]

Seems reasonable to me