Remove sites-as-landing-page feature flag

[mpkelly]

Closes #70297

Proposed Changes

• Remove the sites-as-landing-page feature flag

Testing Instructions

• Make sure all traces of sites-as-landing-page have been removed
• Follow the test plan for Sites: Introduce landing page opt-in banner and make sure the Banner is still shown and that the setting still works

Pre-merge Checklist

• Have you written new tests for your changes?
• Have you tested the feature in Simple (P9HQHe-k8-p2), Atomic (P9HQHe-jW-p2), and self-hosted Jetpack sites (PCYsg-g6b-p2)?
• Have you checked for TypeScript, React or other console errors?
• Have you used memoizing on expensive computations? More info in Memoizing with create-selector and Using memoizing selectors and Our Approach to Data
• Have we added the "[Status] String Freeze" label as soon as any new strings were ready for translation (p4TIVU-5Jq-p2)?
• For changes affecting Jetpack: Have we added the "[Status] Needs Privacy Updates" label if this pull request changes what data or activity we track or use (p4TIVU-ajp-p2)?

Related to #

[matticbot]

Calypso Live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-50646

Jetpack Cloud live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-50646&env=jetpack

[github-actions]

Calypso Live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:commit-fb335bb15205b30f817c15e1682712e3de9348a6

Jetpack Cloud live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:commit-fb335bb15205b30f817c15e1682712e3de9348a6&env=jetpack

[matticbot]

Here is how your PR affects size of JS and CSS bundles shipped to the user's browser:

Sections (~24 bytes removed 📉 [gzipped])

name             parsed_size           gzip_size
root                   -44 B  (-2.5%)       -8 B  (-1.0%)
sites-dashboard        -41 B  (-0.0%)       -7 B  (-0.0%)
account                -41 B  (-0.0%)       -9 B  (-0.0%)

Sections contain code specific for a given set of routes. Is downloaded and parsed only when a particular route is navigated to.

Legend

What is parsed and gzip size?

Parsed Size: Uncompressed size of the JS and CSS files. This much code needs to be parsed and stored in memory.
Gzip Size: Compressed size of the JS and CSS files. This much data needs to be downloaded over network.

Generated by performance advisor bot at iscalypsofastyet.com.

[wojtekn]

Looks good, works as expected.

[vykes-mac]

works as expected

[a8ci18n]

This Pull Request is now available for translation here: https://translate.wordpress.com/deliverables/7653933

Hi @mpkelly, could you please edit the description of this PR and add a screenshot for our translators? Ideally it'd include this string: Admin home

Thank you in advance!

[a8ci18n]

Translation for this Pull Request has now been finished.