Remove inner white border effect on popovers

[andresgalante]

The popovers have an inner white border on the headers, it's a nice effect but I think it makes the styles too opinionated, plus it's a code overhead to generate them and it makes it harder to convert popovers from px to rems.

How about we remove that effect and simplify the popover code?

[mdo]

Yeah, I'm fine nuking them :D.

[andresgalante]

@mdo Cool, I'll send a PR removing them and changing the units to rem today.