Remove old All Newsletters page

@dblatcher has started the work to remove the old page. We should finish this time permitting, it's not high priority.

https://github.com/guardian/frontend/pull/26545

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This PR requires testing but should be complete (it removes the feature switch and clears up the dead code) but we need make sure we are happy to permanently remove this page. One thing to check is that the data team are not seeing a concerning uplift in suspected bot sign-ups on the new page.