docking: Also reduce the app grid area when in auto-hide mode

We now need to also adapt the app-grid area to the available space in
order to correctly allocate the left/right buttons in autohide mode.

LP: #1979096, #1992847

docking.js

@@ -2109,8 +2109,6 @@ var DockManager = class DashToDock_DockManager {
 
                 if (!wsThumbnails && this.mainDock.position === St.Side.BOTTOM)
                     box.y2 -= spacing;
-            } else if (state === OverviewControls.ControlsState.APP_GRID) {
-                return box;
             }
 
             if (this.mainDock.isHorizontal || this.settings.dockFixed)