Remove unnecessary suggestion types in LinkControl

[getdave]

We should extract these types into a CONSTANT as it is used across the entire component.

Over time, I've begun to question whether we even need these. I feel like I may have left a similar comment recently elsewhere, but: For what purpose do we assign type for manually-entered URLs? It seems like a lot of extra work to assign and accommodate, when it may just be enough to assign nothing, and use the absence of a type as an indicator of a value being manually-entered.

isDirectEntry = ! value.type;

Edit: Not suggesting this be done as part of this pull request, but I would consider it as an alternative to a refactoring it.

Originally posted by @aduth in https://github.com/_render_node/MDE3OlB1bGxSZXF1ZXN0UmV2aWV3MzUzMTM3NTAx/pull_request_reviews/more_threads