delete clipboard properties after pasting them

https://tronche.com/gui/x/icccm/sec-2.html#s-2.4 specifies:
> Once all the data in the selection has been retrieved,
> the requestor should delete the property in the SelectionNotify request

Most Clipboard-Owners ignore whether or not the property is already set,
so this is mostly a cosmetic change to keep the windows property list clean.

However, at least synergy decides to wait for the requestor to delete
the properties if they are already set by a previous paste (from synergy).

Signed-off-by: Christoph Lohmann <20h@r-36.net>