appendToBody=true leaves span in body when popper is destroyed

[psalaets]

When the appendToBody prop is true, the popper parent element is being left in body.

Reproduces issue: https://jsfiddle.net/psalaets/g7yg9v7s/1/

In that fiddle, the count should never be > 1 but it increases each time you toggle the popover component because the span is not cleaned up.