Stop spree_sample:load creating a duplicate record

Spree::ShippingCategory.create!(:name => "Default") is run in the following migration https://github.com/spree/spree/blob/master/core/db/migrate/20130830001033_add_shipping_category_to_shipping_methods_and_products.rb therefore a create command here would create a duplicate if a db:migrate had been run prior. shipping_categories.rb could be removed from samples completely but users of db:reset (that uses db:schema:load and loads the db from schema not migrations) would need it to be created here.

Fixes #5161
Revert ReimbursementItem model

This removes ReimbursementItem model and squishes it back into the ReturnItem model.
We split this model out of ReturnItem and we still think that's a good idea.  But we've run into some issues with it so we'll have to look at adding it back in later when we can work through all the details.

Fixes #5160