wayland: Make data_source_target() an empty stub

wayland: Make data_source_target() an empty stub

This request actually means nothing to the upper GDK layers,
we used to preempt a GDK_SELECTION_REQUEST event, but this is too
eager, and not like things work in X11.

Originally in wayland, this event may be used for feedback purposes.
We however don't perform any mimetype-based feedback, so we can
safely ignored.

This makes data_source_send() the only place where we actually
trigger GDK_SELECTION_REQUEST, this one is conceptually the same
than the X11 selection request event.