budgie-popover: Remove evil "margin logic"

As we've now got a fixed popover shape determined at paint-time, there is
no longer any reason to worry about Gtk having a fit and having to paint
*inside* of our own regions to offset theme-differences.
BudgiePopover shape is determined by theme and drawing logic, and will
have an absolute size that it can draw within, including the tail region
and rounded corners.