Revert "Close the split-tab-view drop target when dragging ends prematurely"

This reverts commit fd00a61bbfca5d7616add6dda73dec0de609e32a.

Reason for revert: breaks Wayland drag drop tests

https://ci.chromium.org/ui/p/chromium/builders/ci/linux-wayland-weston-rel-tests/1437/overview

Per agale@, this CL Needs to check if drop_target_view_ is defined since we don't construct it on Wayland

Bug: 433720670
Original change's description:
> Close the split-tab-view drop target when dragging ends prematurely
>
> This CL addresses a bug where ending a drag while the drop target is
> animating-in (but before the cursor is over the drop target) causes the
> drop target view to be stuck visible. The reason for this was that the
> `MultiContentsViewDropTargetController` does not close the target drop
> view for "drag exit" events, and the drop target view isn't handling
> drags yet.
>
> This is fixed by plumbing drag-end events, similar to how drag-exit and
> drag-update events are plumbed.
>
> Change-Id: Icd5c070ec8fafaff71af0c5edd62acc2b6f0cbfd
> Bug: 433720670
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6777771
> Reviewed-by: Alison Gale <agale@chromium.org>
> Commit-Queue: Kaan Alsan <alsan@chromium.org>
> Reviewed-by: Avi Drissman <avi@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#1491573}

Bug: 433720670
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Change-Id: I37698be098ce7da22a8cdd8c85331ef37c5d2f17
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6785093
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Commit-Queue: David Bienvenu <davidbienvenu@google.com>
Reviewed-by: Alison Gale <agale@chromium.org>
Owners-Override: David Bienvenu <davidbienvenu@google.com>
Cr-Commit-Position: refs/heads/main@{#1491636}