Fix: don't initialize workpad from WorkpadApp (elastic#32163)

Closes elastic#32161

All the elements are loaded in the router, so loading the from the component just duplicates the work.

This is actually left-over code from before we had a router. Apparently I missed this lifecycle when adding the router, so it's been an issue in Canvas for quite some time now.

### Stats

Workpad | Previous load time | PR load time
-- | -- | -- 
Elastic{ON} Tour Dallas: Canvas v3 | 10 seconds | 7 seconds
Kibana Canvas - Your data, your way. | 32 seconds | 18 seconds

*Time from when the Canvas loading indicator goes away and the workpad app renders to when the loading indicator stops spinning*

x-pack/plugins/canvas/public/apps/workpad/workpad_app/index.js

@@ -6,7 +6,6 @@
 
 import { connect } from 'react-redux';
 import { compose, branch, renderComponent } from 'recompose';
-import { initializeWorkpad } from '../../../state/actions/workpad';
 import { selectElement } from '../../../state/actions/transient';
 import { canUserWrite, getAppReady } from '../../../state/selectors/app';
 import { getWorkpad, isWriteable } from '../../../state/selectors/workpad';
@@ -24,9 +23,6 @@ const mapStateToProps = state => {
 };
 
 const mapDispatchToProps = dispatch => ({
-  initializeWorkpad() {
-    dispatch(initializeWorkpad());
-  },
   deselectElement(ev) {
     ev && ev.stopPropagation();
     dispatch(selectElement(null));

x-pack/plugins/canvas/public/apps/workpad/workpad_app/workpad_app.js

@@ -15,13 +15,8 @@ export class WorkpadApp extends React.PureComponent {
   static propTypes = {
     isWriteable: PropTypes.bool.isRequired,
     deselectElement: PropTypes.func,
-    initializeWorkpad: PropTypes.func.isRequired,
   };
 
-  componentDidMount() {
-    this.props.initializeWorkpad();
-  }
-
   render() {
     const { isWriteable, deselectElement } = this.props;