[Custom Amounts] Remove edit custom amount by percentage option

[toupper]

Fixes WOOMOB-374

Description

This PR removes the option to edit Custom Amounts via percentage. Instead, we now show the custom amount editor directly, skipping the previous menu sheet.

The reason for this change is that editing the custom amount by percentage can be confusing under the current logic. Specifically, the base amount used for the calculation includes the custom amount itself — which likely doesn’t align with user expectations. We considered subtracting the custom amount from the base amount to resolve this, but that would introduce additional complexity (e.g. tax recalculations). That said, we’re open to implementing it in the future if we see user demand.

Additionally, I renamed a property and updated a comment to better reflect their purpose.

Steps to reproduce

1. Go to Orders
2. Tap on + to create a new order
3. Add a product
4. Add a custom amount via percentage
5. Tap on the custom amount pencil button to edit the custom amount. See that, unlike when we were adding a custom amount, we're not offered now the option to edit it via percentage

Screenshots

Simulator.Screen.Recording.-.iPhone.16.-.2025-06-04.at.15.00.08.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	30257
	Version	PR #15709
	Bundle ID	com.automattic.alpha.woocommerce
	Commit	2b09dfc
	Installation URL	7hrstv6nu1h7g

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

[joshheald]

Works well, thanks for sorting this 😊

[joshheald]

Thanks for renaming this 👍