net/netcheck: adjust HTTPS latency check to connection time and avoid data race

The go-httpstat package has a data race when used with connections that
are performing happy-eyeballs connection setups as we are in the DERP
client. There is a long-stale PR upstream to address this, however
revisiting the purpose of this code suggests we don't really need
httpstat here.

The code populates a latency table that may be used to compare to STUN
latency, which is a lightweight RTT check. Switching out the reported
timing here to simply the request HTTP request RTT avoids the
problematic package.

Fixes tailscale/corp#25095

Signed-off-by: James Tucker <james@tailscale.com>
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