Remove unused var mainCanvas (#488)

Author: huchenlei

src/scripts/app.ts

@@ -1855,10 +1855,6 @@ export class ComfyApp {
     this.canvasEl = canvasEl
     await this.#setUser()
 
-    // Create and mount the LiteGraph in the DOM
-    const mainCanvas = document.createElement('canvas')
-    mainCanvas.style.touchAction = 'none'
-
     this.resizeCanvas()
 
     await Promise.all([