Contact Form: remove old menu image

Jetpack requires 3.8 now, so no need to account for pre-mp6

I tried running `grunt` to automatically update [the RTL file](https://github.com/Automattic/jetpack/blob/master/modules/contact-form/css/rtl/menu-alter-rtl.css), but it seems that the Contact Form CSS files are not included in Grunt right now:
https://github.com/Automattic/jetpack/blob/master/Gruntfile.js#L26