Fix box & violin inner parts removal

[etpinard]

fixes #2777 and a bunch of other restyle/react bugs introduced in #2579, where the box and violin meanlines and points d3-data-binds assume a clean parent <g> and didn't care about clearing parts that should no longer be visible.

cc @alexcjohnson @dmt0

[alexcjohnson]

I like this pattern! But two comments about it:

• trace.box and trace.meanline are deleted rather than left with visible: false - so can't we just say (trace.type === 'violin' && trace.box) etc?
• I'm a bit uneasy relying on uninitialized vars being undefined - seems like it makes things more fragile, for example if we use fn again, or wrap this block in a loop or something... I'd rather a pattern like

var fn;
if(...) fn = Lib.identity;
else fn = []; // or just var fn = []; and drop the else, dunno which is better

var paths = sel.selectAll('path.box').data(fn);

[alexcjohnson]

Perhaps trace.type !== 'violin' || trace.box? NBD either way, we'd of course figure it out pretty quick if we add any other trace type that reuses box drawing.

[alexcjohnson]

Very nicely done - getting us that much closer to regular d3 idioms - and great tests! Both of my comments are nonblocking - 💃