Move {gsDesign2} to Suggests

[jdblischak]

Follow-up to #301

Removes the following NOTE from R CMD check:

❯ checking dependencies in R code ... NOTE
  Namespace in Imports field not imported from: 'gsDesign2'
    All declared Imports should be used.

[jdblischak]

More xref's: #296 (comment), #297 (review)

[LittleBeannie]

Nice catch, thank you, @jdblischak !