🔪 last remaining signs of placeholder key in calcdata

- no need for it anymore.

Author: etpinard

src/plots/plots.js

@@ -2012,11 +2012,8 @@ plots.doCalcdata = function(gd, traces) {
         //
         // This ensures there is a calcdata item for every trace,
         // even if cartesian logic doesn't handle it (for things like legends).
-        //
-        // Tag this artificial calc point with 'placeholder: true',
-        // to make it easier to skip over them in during the plot and hover step.
         if(!Array.isArray(cd) || !cd[0]) {
-            cd = [{x: false, y: false, placeholder: true}];
+            cd = [{x: false, y: false}];
         }
 
         // add the trace-wide properties to the first point,