Remove GraphQL endpoint used in prototype

Once the personalized home page is launched we'll want to remove the unused the GraphQL endpoint implemented in !183872 (merged) as it is only being used by the prototype as noted as part of the discussion in #545446 (comment 2656295919)

As noted in the discussion as well, to keep in mind:

Before removing the existing GraphQL endpoints, consider how we'll need to prototype / implement https://gitlab.com/groups/gitlab-org/-/epics/18357+. That may be more challenging without an available API.