unixsock plugin: Do not unregister a write-callback in us_shutdown().

unixsock plugin: Do not unregister a write-callback in us_shutdown().

The unixsock plugin no longer registers a write-callback. Basically,
unregistering a non-existent callback does not do any harm, However, if no
write-callback has been registered at all, the appropriate linked list has
never been created and the process will segfault when trying to access it.

Signed-off-by: Sebastian Harl <sh@tokkee.org>
Signed-off-by: Florian Forster <octo@huhu.verplant.org>