Remove plyr::compact()

[MichaelChirico]

Another in the series (#370, #371) of gradually weaning off of {plyr} as a dependency.

"Modern" code I suppose is meant to use purrr::compact() instead, but it's not a drop-in replacement:

x = list(character(), NULL, 1)
purrr::compact(x)
# [[1]]
# [1] 1

plyr::compact(x)
# [[1]]
# character(0)

# [[2]]
# [1] 1