Just make [relayManager] always handle it, there's no benefit to
checking bestAddr's.
Also, remove passing of disco.Pong to [relayManager] in
endpoint.handlePongConnLocked(), which is redundant with the callsite in
Conn.handleDiscoMessage(). Conn.handleDiscoMessage() already passes to
[relayManager] if the txID us not known to any [*endpoint].
Updates tailscale/corp#27502
Signed-off-by: Jordan Whited <jordan@tailscale.com>