[Flexiglass] Remove Modifier.stackVerticalOverscroll

Since the introduction of OffsetOverScrollEffect, there is no need to
create our own solution for Notifications, as OffsetOverScrollEffect is
already being added to any element that's using Modifier.verticalScroll.

Bug: 360100111
Test: chech the overscroll effect of Notifications
Flag: com.android.systemui.scene_container
Change-Id: I3d09709a66ced5dcf897ba03fc973d66fe748ca8