remove selected exports

I noticed that many functions which appear to be internal helper functions had the `@export` roxygen keyword attached to them. I took a stab at removing the functions which I believed didn't require exporting and tested with the networkD3 and dygraphs packages (at the console, within an Rmd, and within a Shiny application) and found no issues. @jcheng5, @ramnathv, and @yihui could you confirm from your end that these functions aren't required outside of the package?