[Woo POS][Coupons] Remove searchCouponsInPOS flag

[iamgabrielma]

Addition to WOOMOB-613 and continuation of #15751

Description

This PR cleans up the searchCouponsInPOS feature flag.

Testing information

In POS ( now in a new tab!) there should be no difference on how the coupons search button is rendered, it always appear when we navigate to the Coupons header, and searches for coupons on usage.

[dangermattic]

	1 Warning
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[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	30491
	Version	PR #15761
	Bundle ID	com.automattic.alpha.woocommerce
	Commit	9cf0987
	Installation URL	58kul83vo7ujo

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

[staskus]

🚀