Remove toasts from DOM after hiding them

Created by: MartijnCuppens

From https://github.com/twbs/bootstrap/pull/27792#issuecomment-445064288: We also do this with the alerts. The problem with not removing them is that there's a margin above the second toast if we dismiss the first: https://deploy-preview-27792--twbs-bootstrap4.netlify.com/docs/4.1/components/toasts/#stacking