workspaceThumbnail: Destroy thumbnails on destruction

Often, when using multiple monitors, the thumbnails box may be
destroyed before the 'hidden' signal of the Overview is emitted,
which leaves a bunch of lingering signal connections to the
workspace manager.

Call this._destroyThumbnails() in the _onDestroy handler, which
disconnects from workspace manager. This fixes the some of the log
spam that GNOME Shell produces with backtraces of destroyed actors.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1860>
(cherry picked from commit e9119c62)