wayland: Rely on MetaSelection::owner-changed for .selection event emission

We already have a signal callback that translates selection ownership changes to data_device/primary .selection events. Given both will be run when a data source is being replaced, and this event emission being deleted is kinda short sighted in that in only knows about Wayland, rely entirely on MetaSelection::owner-changed emission.

Fixes spurious .selection(null) events being send when a compositor-local source takes over the selection without the focus changing (eg. screenshot to clipboard).

Closes: #1160 (closed)