GtkWindow: Don't double free export user data

GtkWindow: Don't double free export user data

The user data passed when exporting a Wayland window was supposed to be
freed using the destroy_func, as is commonly done. This was previously
broken, as the user data was just NULL:ed when exported, and only
actually destroyed when unexporting before having exported.

While e016d9a5dba6f6f99aee94d0b72c00bee299b96a fixed this, it introduced
a regression, as GtkWindow was nice enough to free the memory anyway
after having received the exported handle, causing it now to double
free.

https://bugzilla.gnome.org/show_bug.cgi?id=782109