Remove Alamofire as a library dependency

[crazytonyli]

Description

The apps has been running with useURLSession turned on for a couple of weeks. See wordpress-mobile/WordPress-iOS#22540 and woocommerce/woocommerce-ios#11927. There is no major issue reported. And it's likely we'll fix bugs in the new implementation, instead of reverting back to the old one.

This PR removes Alamofire as a dependency to the library, but it's still used in the unit tests to verify some encoding request encoding implementations, i.e. multipart-form.

Testing Details

Changes are covered by unit tests.

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[mokagio]

🎉

This PR removes Alamofire as a dependency to the library, but it's still used in the unit tests to verify some encoding request encoding implementations, i.e. multipart-form.

I hadn't considered the option of keeping it in the unit tests temporarily. It's a great idea, because it lets us focus on removing Alamofire from the production code faster. 👍