remove list_values() from API

[PietrH]

          `list_values` doesn't use a database connection, thus doesn't need any changes to be used over the api. I'm not sure it makes sense to provide an API version of this function. 

#' List all unique values from a data.frame column
#'
#' Get a vector with all unique values found in a given column of a data.frame.
#' Concatenated values (`A,B`) in the column can be returned as single values
#' (`A` and `B`).

Originally posted by @PietrH in #26 (comment)