CLEANUP: proxy: detach the name node in proxy_free_common() instead

CLEANUP: proxy: detach the name node in proxy_free_common() instead

This changes commit d2a9149f0 ("BUG/MINOR: proxy: always detach a proxy
from the names tree on free()") to be cleaner. Aurélien spotted that
the free(p->id) was indeed already done in proxy_free_common(), which is
called before we delete the node. That's still a bit ugly and it only
works because ebpt_delete() does not dereference the key during the
operation. Better play safe and delete the entry before freeing it,
that's more future-proof.