relay: Don't make DNS timeout trigger an overload

Tor has configure libevent to attempt up to 3 times a DNS query for a
maximum of 5 seconds each. Once that 5 seconds has elapsed, it consider
the query "Timed Out" but tor only gets a timeout if all 3 attempts have
failed.

For example, using Unbound, it has a much higher threshold of timeout.
It is well defined in
https://www.nlnetlabs.nl/documentation/unbound/info-timeout/ and has
some complexity to it. But the gist is that if it times out, it will be
much more than 5 seconds.

And so the Tor DNS timeouts are more of a "UX issue" rather than a
"network issue". For this reason, we are removing this metric from the
overload general signal.

See https://gitlab.torproject.org/tpo/network-health/team/-/issues/139
for more information.

Fixes #40527

Signed-off-by: David Goulet <dgoulet@torproject.org>

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