Remove legacy server side AB test framework

[DanielCliftonGuardian]

What is the value of this and can you measure success?

Removes the old server-side AB testing framework which has been superseded.
The old framework had no active experiments so it was already a no-op.

What does this change?

Deleted core framework and related code.

I have moved ContentPresser feature switch to FeatureSwitches from the now deleted ServerSideExperimentSwitches

Warning

This PR drops the abTests field from the DCR payload, which DCR currently lists as required.
Do not deploy to PROD until the companion dotcom-rendering change (make abTests optional,
remove from schema required[], stop reading it) is live in PROD.

Deployment steps:

1. DCR PR: abTests optional + de-referenced
2. This PR
3. DCR PR: delete the now-unused optional abTests property + schema definition

[github-actions]

Deploy build 7202 of dotcom:frontend-all to CODE

All deployment options

• Deploy build 7202 of dotcom:frontend-all to CODE
• Deploy parts of build 7202 to CODE by previewing it first
• What's on CODE right now?

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From guardian/actions-riff-raff.