Activate persistent selection for ternary plots [1852]

- Ternary was the only base plot with inconsistent behavior in terms
  of holding the Shift key. It allowed to temporarily pan (zoom when
  in pan) in zoom, lasso and select drag modes.

Author: rmoestl

src/plots/ternary/ternary.js

@@ -464,10 +464,6 @@ proto.initInteractions = function() {
             dragOptions.xaxes = [_this.xaxis];
             dragOptions.yaxes = [_this.yaxis];
             var dragModeNow = gd._fullLayout.dragmode;
-            if(e.shiftKey) {
-                if(dragModeNow === 'pan') dragModeNow = 'zoom';
-                else dragModeNow = 'pan';
-            }
 
             if(dragModeNow === 'lasso') dragOptions.minDrag = 1;
             else dragOptions.minDrag = undefined;