Rework the last delsel/electric fix.

Rework the last delsel/electric fix.
* lisp/delsel.el (delete-selection-helper): Use a function instead of a hook.
(delete-selection-pre-hook): Use use-region-p.
(delete-selection-self-insert-function): Remove.
(self-insert-command): Obey self-insert-uses-region-functions.
(self-insert-iso): Revert to previous setting, since we don't actually
know what that command does.
(delete-selection-self-insert-hooks): Remove.
* lisp/electric.el (electric-pair-delete-selection-self-insert-function):
Rename to electric-pair-will-use-region, return a boolean.
(electric-pair-mode): Adjust accordingly.  Don't require delsel.