emoji: Remove bottom shadow on the emoji picker scrollable

We don't have other scrollables that are vertical, touching the bottom
viewport edge, and have shadow there.

This was brought up by Chris:

> The shadow effect makes the viewport edge more graceful when there's
  not already a clear boundary that coincides with it, but the screen
  edge is such a boundary. Similarly the top edge of the keyboard when
  that's open. (Clear definition of the top edge of the keyboard is
  the platform's job, not ours.)

CZO discussion:
  https://chat.zulip.org/#narrow/channel/530-mobile-design

Signed-off-by: Zixuan James Li <[email protected]>

Author: PIG208

Diff for: lib/widgets/emoji_reaction.dart

@@ -530,7 +530,7 @@ class _EmojiPickerState extends State<EmojiPicker> with PerAccountStoreAwareStat
               style: const TextStyle(fontSize: 20, height: 30 / 20))),
         ])),
       Expanded(child: InsetShadowBox(
-        top: 8, bottom: 8,
+        top: 8,
         color: designVariables.bgContextMenu,
         child: CustomScrollView(
           slivers: [