Revert "Activate dark theme when the UiMode is UI_MODE_NIGHT_YES."

This reverts commit a5c846c3.
Test: switch wallpapers, observe theme
Bug: 110758454
Reason: Obsolete in Q. UiMode is the only flag that drives
        the dark ui.

Change-Id: Ie0307336b8c3896e3ecf934e073f51382c26364e