[SB] Remove unused StatusBarIconController#setIconA11yLiveRegion.

This method was used by the old screen sharing indicator, but that
indicator was removed in I0c7426924b1c3770efc29a916399780112c78a9c
and no other icons use accessibilityLiveRegion.

Bug: N/A
Flag: EXEMPT removing unused code
Test: m SystemUI
Change-Id: I8d284f63a0ece1179c8fb4c51bfd2920799c974b