Remove box ungrouping magic

[etpinard]

fixes #3390 as per @nicolaskruchten 's #3390 (comment)

To be merged into #3234 to avoid conflicts in box/cross_trace_calc.js

cc @plotly/plotly_js

[antoinerg]

@etpinard I am not too familiar with box trace but I am a bit confused by the result of the new baseline you introduced in this PR. According to the description of boxmode:
If *group*, the boxes are plotted next to one another centered around the shared location.. If I only have one box at a given location, I would expect the shared location to be the box's center position. Yet, the boxes in baseline box_single-group.json are not aligned anymore with the tick labels 🤔 Is this normal?

[etpinard]

I would expect the shared location to be the box's center position. Yet, the boxes in baseline box_single-group.json are not aligned anymore with the tick labels Is this normal?

See discussion in #3390 (comment)

[antoinerg]

See discussion in #3390 (comment)

I think I understand after a careful second read. I agree, let's 🔪 that magic!

💃