Get rid of unnecessary KEY_RESTRICT_BACKGROUND preference.

This preference is not needed for Data plan settings page. This change
fixes the NPE which happens because we try to remove a preference which
is not added to the settings preference fragment.
Test: make RunSettingsRoboTests -j40
Bug: 64068792

Change-Id: Ibbcf1637b8aed1a94542ea3786170f2f318df4a9