Spree::Variant override tax_category association

[rioug]

What we should change and why (this is tech debt)

When an instance set a default tax category, Spree::Variant will override the tax_category association and return the default one if none are set on the variant.

openfoodnetwork/app/models/spree/variant.rb

Lines 172 to 174 in 2adb69e

	def tax_category
	super || TaxCategory.find_by(is_default: true)
	end

This is confusing as it doesn't reflect the data in the database, and "None" (as in none set) is valid tax category.
We should remove this override and fix the underlying data.

Fix

Currently only ca_prod, fr_prod, uk_prod, nz_prod, in_prod have a default tax category set. Number of variants with no tax category for each instance :

• ca_prod 28886
• fr_prod 44277
• in_prod 15
• nz_prod 2309
• uk_prod 126871

We will need to migrate all these variants to the default tax category.
Then we can remove the association override, and we should update the "Add new product" UI to select the default tax category when there is one set. This should get us close to the current behaviour.

Context

This came up in PR: #12333 , more context #12333 (comment)