Remove Google Analytics from rust docs

[nuke-web3]

Originally posted by @imadarai in #10007 (comment)

The only remaining old devhub links are in https://github.com/paritytech/substrate/blob/master/.maintain/rustdoc-header.html I am asking @imadarai to check in on if these need to be migrated, can be added into this PR or a followup.

---

Does this header gets applied to all the rust-docs or just substrate?

I'd like to remove Google Analytics and update the script with Simple Analytics (cookie-less), but will need the exact links where it's being used.

This is unclear to me - what steps need to be taken to move forward?

[nuke-web3]

The file can be removed as the style links are broken now too. Unless the team explicitly relies on this for whatever reason, and this line as well:

substrate/.maintain/rustdoc-header.html

Lines 1 to 10 in 969a70d

	<script defer type="text/javascript" src="https://substrate.dev/js/config.js"></script>
	<script defer type="text/javascript" src="https://substrate.dev/js/klaro.min.js"></script>
	<script async type="opt-in" data-type="application/javascript"
	data-src="https://www.googletagmanager.com/gtag/js?id=UA-145158313-2" data-name="googleAnalytics"></script>
	<script type="opt-in" data-type="application/javascript" data-name="googleAnalytics">
	window.dataLayer = window.dataLayer || [];
	function gtag(){dataLayer.push(arguments);}
	gtag('js', new Date());
	gtag('config', 'UA-145158313-2');
	</script>

substrate/.gitlab-ci.yml

Line 664 in 969a70d

- RUSTDOCFLAGS="--html-in-header $(pwd)/.maintain/rustdoc-header.html"