Remove overflow indicator for ime menu

Required to remove unnecessary white lines above and below
the scroll content.

Bug: 1121370
Change-Id: I8628085150dbc5f4dd548fe9488e8fc88d8b6912
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2373233Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Ahmed Mehfooz <amehfooz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#801203}

ash/system/ime_menu/ime_menu_tray.cc

@@ -299,7 +299,6 @@ class ImeMenuListView : public ImeListView {
     // DetailedViewDelegate:
     void TransitionToMainView(bool restore_focus) override {}
     void CloseBubble() override {}
-    bool IsOverflowIndicatorEnabled() const override { return true; }
 
    private:
     DISALLOW_COPY_AND_ASSIGN(Delegate);