clipboard: remove dead code

clipboard: remove dead code

When setting contents of the clipboard and ownership or user data changes, we
end up calling clipboard_unset() to fully cleanup the previous clipboard state.
This call will itself call clear_func() for the previous user_data, and always
reset both 'get_func' and 'clear_func' to NULL.

So it's actually not possible to have 'get_func' being non-NULL once we have
called clipboard_unset(), so just remove that condition check and the code
inside.

https://bugzilla.gnome.org/show_bug.cgi?id=694924