Dedup onClickListener for clearing silent notifs

Removed code is no op;
NotificationSectionsManager #onClearSilentNotifsClickListener is unused.

NSSLC does what this means to do, via
    mSilentHeaderController.setOnClearAllClickListener

Bug: 175410557
Test: clear notifs in silent section; no regressions

Change-Id: I553196167c5d1f422086d55e079af65eeff8a8c2