make consistent use of url() in css files

When using url() in CSS files, you don't need to add the optional ' or ". More info.

Most of core doesn't use them, some does, this patch removes them all since they aren't needed. it is also best practice not to use them, most top CSS people don't. This was a habit that was carried over as including them lead to some browser bugs. Those no longer exist, but the practice still holds.

Simple patch, should be good.