Remove functions/arguments deprecated in purrr 0.3.0

[hadley]

Some of these hung on because we didn't properly flag as deprecated, but I think it's time to let them go. Hopefully the revdep checks support this decision.

@shikokuchuo if we're going to make it a minor release, we can advance some deprecations as well.

Revdep check failures:

• [ ] https://github.com/cran/CPAT — uses partial(.lazy)
• [ ] https://github.com/EagerAI/kerastuneR — tidyjson imports at_depth
• [ ] https://github.com/andrewhooker/PopED — false positive (package size)
• [ ] https://github.com/ramiromagno/quincunx — tidyjson imports at_depth
• [ ] https://github.com/theme-ontology/stoRy — imports at_depth
• [ ] https://github.com/colearendt/tidyjson — imports at_depth