Added MutableAclProvider::deleteSecurityIdentity

This provides a very simple function to enable the deletion of a SecurityIdentity.

Developers can add a listener on the delete of a user and remove all the related ACLs.
Foreign keys already ensure that the ACEs are properly deleted.

Among the problems of not deleting the SecurityIdentity:

* Inconsistent database, referring to a non-existent user.
* If a user is deleted and another is created with the same name, it will inherit all the old user’s ACEs

Not addressed by this PR: Changing a user’s username breaks the related ACLs. See #5787

See also: https://groups.google.com/forum/#!topic/symfony2/mGTXlTWiMs8/discussion