Fix crash in holding space when dragging items.

Previously we removed all holding space item views when closing the
holding space tray bubble. This was due to lifecycle concerns since it
is possible for holding space item views to outlive their delegate due
to the holding space bubble widget being asynchronously destroyed.

Removing child views during a destruction sequence can have unintended
side effects as it can trigger re-layout, as was the case in this crash.

This CL removes the logic which removed child views when closing the
bubble widget as item views only interact with the delegate in response
to UI events which should not occur when the widget is being closed.

Bug: 1143426
Change-Id: Ie0548401b8672c2c0c3b571e45c80900edd53b1d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2506513
Reviewed-by: Toni Baržić <tbarzic@chromium.org>
Commit-Queue: David Black <dmblack@google.com>
Cr-Commit-Position: refs/heads/master@{#821999}