cnp: remove loss callback when object is deleted

cnp: remove loss callback when object is deleted

Summary:
SEG_FAULT happens when the object which has selection is deleted,
and new selection is done at another object.
Reason: loss_cb is not removed when the object which has selection is deleted.
When new selection is set for new object, the loss_cb is called for deleted
object. As result, SEG_FAULT happens.

Test Plan:
Run elementary test, open Entry, do selection,
close Entry window, open Entry one more time, do selection.

Reviewers: JackDanielZ

Differential Revision: https://phab.enlightenment.org/D2763