gdtools dependency/suggests no more needed

[davidgohel]

Hello,

The package suggests and use gdtools because it uses vdiffr, vdiffr is not using gdtools anymore, I think it should be removed.

(I can PR if you agree :) )

KR
David

[davidgohel]

gdtools::set_dummy_conf() will be deprecated and defunct in the following release of gdtools. It is an empty function since years and it does not make sense to maintain it anymore