respect width/height (no fullscreen) if set in electronOptions... (#3187)

... in `config.js`.

Solves #3174 

With getting width/heigt from
`electron.screen.getPrimaryDisplay().workAreaSize` introduced with
https://github.com/MichMich/MagicMirror/pull/3161 the solution was to
remove the `setFullscreen` line.

So per default the fullscreen resolution is used but when someone now
uses `electronOptions.width`/`electronOptions.height` in `config.js`
these parameters are used and so "no fullscreen" is possible.

js/electron.js

@@ -130,7 +130,6 @@ function createWindow() {
 	});
 
 	mainWindow.once("ready-to-show", () => {
-		mainWindow.setFullScreen(true);
 		mainWindow.show();
 	});
 }