dnd: Drop captured event handler

dnd: Drop captured event handler

This was here exclusively to silence out events from other pointing
devices in the stage. Since ClutterGrab being used now is global to
all devices and events are coerced to an invisible actor, there is
no need to explicitly do this.

Also, this event handler was set on the stage, while the grab happened
on a child, so it was fairly uneffective already.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2146>