Remove Periphery step from Buildkite pipeline

[RafaelKayumov]

Description

As discussed: p91TBi-dwL-p2#comment-14434 - removing the Periphery from Buildkite pipeline.

• Remove the "Detect unused code" step from .buildkite/pipeline.yml
• Remove DerivedData/Index.noindex/DataStore/export from .buildkite/commands/build-for-testing.sh
• Keeps run / setup scripts for Periphery for future re-use.

Test Steps

Just make sure the buildkite pipeline works fine and there is no Detect unused code step in output log.

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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	pr16387-c0e1576
	Version	23.7
	Bundle ID	com.automattic.alpha.woocommerce
	Commit	c0e1576
	Installation URL	0r68q13nh3vmg

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

[RafaelKayumov]

CC @AliSoftware

[AliSoftware]

👍

PS: Don't hesitate to ping AppsInfra when you'll start on making those run on scheduled jobs instead.

If you feel inclined, feel free to create the .buildkite/run-periphery.yml pipeline yourself with the necessary steps (to build + call run-periphery.sh) and test it via manually-triggered builds on Buildkite (setting PIPELINE=run-periphery.yml in the options dropdown after clicking Manual Build), then we can help in creating a schedule to run that pipeline at specific times in our infra.