dnd: Remove gdk_content_provider_new_with_callback()

dnd: Remove gdk_content_provider_new_with_callback()

Content providers are meant to be immutable, apart from very special
cases, but in those cases they need to emit
gdk_content_provider_content_changed().

Having a constructor that just uses a get_func invites abuse of this
by not making developers aware of those requirments.
In fact, all users in GTK failed to do this.

Instead, code should use the GtkDragSource::prepare signal to create
content providers when needed.

The same problem exists with gdk_content_provider_new_with_formats(),
but this commit doesn't touch that.