DOC: remove text about extensions/helpers
DOC: remove text about extensions/helpers

We do not ship them (yet)

tacaswell/scikit-xray,scikit-xray/scikit-xray,Nikea/scikit-xray,tacaswell/scikit-beam,ericdill/scikit-xray,Nikea/scikit-xray,tacaswell/scikit-xray,tacaswell/scikit-beam,danielballan/scikit-xray,ericdill/scikit-xray,ericdill/scikit-xray,Nikea/scikit-xray,tacaswell/scikit-beam,danielballan/scikit-xray,danielballan/scikit-xray,scikit-xray/scikit-xray,tacaswell/scikit-xray,scikit-xray/scikit-xray