Remove legacy GraphQL Playground plugin from repository (#7050)

We published a standalone AS4-compatible
`@apollo/server-plugin-landing-page-graphql-playground` plugin once as
part of the AS4 release process, but we do not plan to maintain this
further. (If we need another publish, we can restore the source into
this repository or a new repository.)

We do continue to test the plugin in the integration test suite, so we
don't break it.

.changeset/lucky-spies-decide.md

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