chore: drop deprecated context member

This removes the long deprecated `import.parse.complete` member `context`.

BREAKING CHANGES:

* `import.parse.complete` member `context` removed. Use the actual
  event to retrieve error, warning, and references information.

lib/BaseViewer.js

@@ -188,28 +188,7 @@ BaseViewer.prototype.importXML = async function importXML(xml, bpmnDiagram) {
   const self = this;
 
   function ParseCompleteEvent(data) {
-
-    const event = self.get('eventBus').createEvent(data);
-
-    // TODO(nikku): remove with future bpmn-js version
-    Object.defineProperty(event, 'context', {
-      enumerable: true,
-      get: function() {
-
-        console.warn(new Error(
-          'import.parse.complete <context> is deprecated ' +
-          'and will be removed in future library versions'
-        ));
-
-        return {
-          warnings: data.warnings,
-          references: data.references,
-          elementsById: data.elementsById
-        };
-      }
-    });
-
-    return event;
+    return self.get('eventBus').createEvent(data);
   }
 
   let aggregatedWarnings = [];