[Order creation] Remove side by side order form feature flag

[joshheald]

Description

This flag has been enabled for ages, and unfortunately it wasn't very cleanly used, so this change set is bigger than I'd want it to be!

Steps to reproduce

1. Launch the app and open Orders
2. Tap Create and check that the screen works as expected
3. Repeat with editing.

This should be tested on iPad and iPhone, as the presentation is different between them.

Testing information

I've tested on iPad (iOS 17), and iPhone (iOS 18)

Screenshots

side-by-side-order-creation.mp4

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• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

[wpmobilebot]

📲 You can test the changes from this Pull Request in WooCommerce iOS Prototype by scanning the QR code below to install the corresponding build.

	App Name	WooCommerce iOS Prototype
	Build Number	30563
	Version	PR #15775
	Bundle ID	com.automattic.alpha.woocommerce
	Commit	66a4508
	Installation URL	1gtcldidlpfjg

Automatticians: You can use our internal self-serve MC tool to give yourself access to those builds if needed.

[iamgabrielma]

Works well! Tested in iPad iOS17, and iPhone iOS17