Fix scatter3d scalar hovertext handling

[etpinard]

Easy fix cc @alexcjohnson

But: wow, I wish hover from subplot type to another other would be done in a DRYer way 😫

[alexcjohnson]

yep, at some point we should make an attempt to 🌴 it up - will probably uncover and fix a bunch more bugs that way like I did (am doing...) with colorbars in #2681

But nice quick fix 💃

[etpinard]

we should make an attempt to it up

To me, the first step to 🌴 ing this up would be to make all hover-related routines use just _fullData (i.e. no calcdata) so that e.g. scattergl and scatter can share the same hoverPoints method.