Remove unused LOCK_SCREEN_SHOW_SILENT_NOTIFICATIONS usage

ag/19735654 removed the code that read the cached setting value, but
not the field that stored it or the code that wrote it.

Bug: 252815574
Test: atest NotificationLockscreenUserManagerTest
Change-Id: Ie88d7038237ec1d4cbd05591a48005fb34edc566
1 file changed