PolkitAgentSession: fix race between child and io watches

The helper flushes and fdatasyncs stdout and stderr before terminating
but this doesn't guarantee that our io watch is called before our
child watch. This means that we can end up with a successful return
from the helper which we still report as a failure.

If we add G_IO_HUP and G_IO_ERR to the conditions we look for in the
io watch and the child terminates we still run the io watch handler
which will complete the session.

This means that the child watch is in fact needless and we can remove
it.

https://bugs.freedesktop.org/show_bug.cgi?id=60847