Remove payouts overview feature flag

[joshheald]

Description

Payout summaries for WooPayments have been in the app for a long time – this PR belatedly removes the feature flag.

Steps to reproduce

1. Launch the app and select a store with WooPayments
2. Open the Payments menu – observe that the payouts (funds) summary is available at the top. You're unlikely to actually have a payout on a test store though, just numbers in the funds boxes.

Testing information

Screenshots

payout-summary.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	30618
	Version	PR #15778
	Bundle ID	com.automattic.alpha.woocommerce
	Commit	2c182ce
	Installation URL	39eo8r7s6d0p8

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

[iamgabrielma]

🚀