webagg: Don't send figure pixel ratio too early.

This is already done in the WebSocket's `onopen` function, and sending
it during the object's initialization crashes due to the WebSocket not
being ready.

Fixes matplotlib#19129.

Author: QuLogic

lib/matplotlib/backends/web_backend/js/mpl.js

@@ -157,9 +157,6 @@ mpl.figure.prototype._init_canvas = function () {
         1;
 
     this.ratio = (window.devicePixelRatio || 1) / backingStore;
-    if (this.ratio !== 1) {
-        fig.send_message('set_dpi_ratio', { dpi_ratio: this.ratio });
-    }
 
     var rubberband_canvas = (this.rubberband_canvas = document.createElement(
         'canvas'