Removes useless uses of the slice mixin

[nicolasbadia]

No description provided.

[mauritslamers]

I am not sure this is the right approach. The advantage of using @include slice is that the BT currently converts it into base64 encoded urls. If sc_static is used, there is no way of embedding the icons. Moreover, it might be clever to take a look at how we could get rid of these files by using FontAwesome.

[nicolasbadia]

I believe replacing this images by FontAwesome icons is indeed the way to go.
I also think we should add an sc method to the BT to specifically convert images into base64 encode urls. We should also deprecate slice as you suggested on IRC as it doesn't really makes sense anymore in the modern web.