BUG/MEDIUM: fd: Don't modify the update_mask in fd_dodelete().

Only the pollers should remove bits in the update_mask. Removing it will
mean if the fd is currently in the global update list, it will never be
removed, and while it's mostly harmless in 1.9, in 1.8, only update_mask
is checked to know if the fd is already in the list or not, so we can end
up trying to add a fd that is already in the list, and corrupt it, which
means some fd may not be added to the poller.

This should be backported to 1.8.

src/fd.c

@@ -371,7 +371,6 @@ static void fd_dodelete(int fd, int do_close)
 	port_range_release_port(fdinfo[fd].port_range, fdinfo[fd].local_port);
 	fdinfo[fd].port_range = NULL;
 	fdtab[fd].owner = NULL;
-	fdtab[fd].update_mask &= ~tid_bit;
 	fdtab[fd].thread_mask = 0;
 	if (do_close) {
 		polled_mask[fd] = 0;