Revert "Do not execute overwritten bindings"

Revert "Do not execute overwritten bindings"

This reverts commit 639208cc7f3ab3d8356363559e8fcf168e32cf0b.

There were two problems with this submit:
    1. Maintaining a hash of all the properties we have assigned bindings
       to is massively inefficient.
    2. The autotest was in the qquickbinding testcase which is for the
       QtQuick "Binding" element, not for generic binding tests.

Change-Id: Id2150dbfe86c6844cc0b115d7f941ae8d6a60643
Reviewed-by: Roberto Raggi <roberto.raggi@nokia.com>