Remove 3.x series deprecated code

[kennyadsl]

We are approaching the release of Solidus 4.0.

That version will be as similar as possible to the last of the 3.x series to reduce the number of breaking changes we introduce. One of the major differences will be removing all the deprecated code.

Assuming people stopped using the code deprecated, this will be the part of the update that won't break any application.

Here's the list of PRs that remove deprecated code:

• Remove solidus_frontend from the meta gem #5026
• Remove deprecated order updater promotions code #4890
• Remove support for deprecated promo rules matching policy #5019
• Remove Deprecated code from Core component #4989
• Remove Deprecated code from API component #5020
• Remove Deprecated code from Backend component #5021
• Remove Deprecated Preferences #5022
• Remove deprecated factories usage #5023 + Remove deprecated spree/testing_support file #5063
• Remove support for legacy event system #5024