Deprecate Settings.System.SHOW_WEB_SUGGESTIONS

In earlier Android versions, Quick Search Box set this setting,
and the browser and QSB read it. Now the Browser has stopped
using it, and QSB has been unbundled and removed from the system
settings UI.

Applications that show web suggestions should have their own settings
to control it instead.

Bug: 3021480
Change-Id: I4e62bf26944287f804e50eb93843484a0356fffb