CrOS Shelf: Clean up an already-addressed TODO

Change-Id: I9ba9016fdf48fab67a929cbb49ca380045a0fb92
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1832981
Commit-Queue: Manu Cornet <manucornet@chromium.org>
Commit-Queue: Matthew Mourgos <mmourgos@chromium.org>
Auto-Submit: Manu Cornet <manucornet@chromium.org>
Reviewed-by: Matthew Mourgos <mmourgos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#701409}

ash/shelf/shelf_config.cc

@@ -124,9 +124,6 @@ void ShelfConfig::OnOverviewModeEnded() {
 }
 
 int ShelfConfig::shelf_size() const {
-  // TODO(manucornet): Add unit tests to check that this returns the correct
-  // sizes in various situations.
-
   // Before the hotseat redesign, the shelf always has the same size.
   if (!chromeos::switches::ShouldShowShelfHotseat())
     return 56;