fires closed.bs.alert *after* DOM detach()

fires closed.bs.alert *after* element is removed from DOM.
Previously it fired while the element was still attached.
Update alert.js

Added call to remove() after event fires to clean up data as per discussion at #12379
Merge pull request #13406 from EnsignR/closed.bs.alert-after-remove

fires closed.bs.alert *after* DOM detach() as per #12379