Remove unnecessary spacer

Now that the hub properly handles overscroll, we remove this spacer to
let all touches go to the grid. The grid will then send any touches it
doesn't consume up to the STL for scene transitions.

Bug: 346330753
Test: verified on hub with < 6 widgets where there is no scrolling
Test: verified on hub with > 6 widgets where the grid scrolls
Flag: com.android.systemui.communal_hub
Change-Id: Ice39f71ccad9145ce3a20f542a5b494b9798428f