Change Element.removeData to actually remove keys from data map

Without this change, it is very easy for an application to slowly leak memory as Raphael continues to add new keys to `eldata`.

This memory leak turns out to be a problem for [Piwik](https://piwik.org/), which uses
kartographer.js (which uses raphael) to draw an SVG image of the United States
in its analytics dashboard ([see demo](https://demo.piwik.org/index.php?module=CoreHome&action=index&idSite=3&period=day&date=yesterday)).

Every time the dashboard is redrawn, it spawns ~700 new Elements with data. [I've submitted a pull request that appropriately cleans up this data when the user navigates away from the page using `removeData`](matomo-org/matomo#11350), but Raphael leaves an empty object in the `eldata` map keyed on the element's old ID. Since kartographer/Piwik does not reuse IDs, `eldata` will grow continually on each redraw until the application is out of memory.

dev/raphael.core.js

@@ -3120,7 +3120,7 @@ define(["eve"], function(eve) {
     \*/
     elproto.removeData = function (key) {
         if (key == null) {
-            eldata[this.id] = {};
+            delete eldata[this.id];
         } else {
             eldata[this.id] && delete eldata[this.id][key];
         }