Use the default DMG background

Don't bother with carefully positioning for the custom
background - a simple clear default is just fine.

Author: pimterry

package.json

@@ -61,7 +61,6 @@
       "forceCodeSigning": false
     },
     "dmg": {
-      "background": "src/dmg-background.png",
       "artifactName": "HttpToolkit-${version}.dmg"
     },
     "win": {