refactor: remove unnecessary get_connect(), get_content(), get_variant() R6 methods

[nealrichardson]

Intent

Just access the R6 slots directly. Remove ambiguity about which pattern we want to use (#459 (comment))

Approach

Find and replace, with some careful reading to distinguish some confusing cases:

• Where get_connect() was called on a Connect object itself (it just returned self!) rather than some API entity
• Disambiguating the get_content() top-level function that retrieves all content records, vs. the $get_content() R6 method.

Checklist

• Does this change update NEWS.md (referencing the connected issue if necessary)?
• Does this change need documentation? Have you run devtools::document()?

[toph-allen]