change top-paper <defs> id to 'topdefs' + uid:

- that way ids remain unique on page
- rm special <defs> merge block in to-svg step

Author: etpinard

src/plot_api/plot_api.js

@@ -2586,7 +2586,7 @@ function makePlotFramework(gd) {
         .attr('id', 'defs-' + fullLayout._uid);
 
     fullLayout._topdefs = fullLayout._toppaper.append('defs')
-        .attr('id', 'defs-' + fullLayout._uid);
+        .attr('id', 'topdefs-' + fullLayout._uid);
 
     fullLayout._draggers = fullLayout._paper.append('g')
         .classed('draglayer', true);

src/snapshot/tosvg.js

@@ -89,15 +89,7 @@ module.exports = function toSVG(gd, format) {
     // assumes everything in toppaper is a group, and if it's empty (like hoverlayer)
     // we can ignore it
     if(fullLayout._toppaper) {
-        var topDefs = fullLayout._topdefs.node().childNodes;
 
-        for(i = 0; i < topDefs.length; i++) {
-            var topDef = topDefs[i];
-
-            fullLayout._defs.node().appendChild(topDef);
-        }
-
-        fullLayout._topdefs.remove();
 
         var topGroups = fullLayout._toppaper.node().childNodes;