tailcfg: kill off rest of HairPinning symbols

It was disabled in May 2024 in #12205 (9eb72bb51).

This removes the unused symbols.

Updates #188
Updates tailscale/corp#19106
Updates tailscale/corp#19116

Change-Id: I5208b7b750b18226ed703532ed58c4ea17195a8e
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/ipnlocal/c2n_test.go

@@ -324,7 +324,6 @@ func TestRedactNetmapPrivateKeys(t *testing.T) {
 		f(tailcfg.Location{}, "Priority"):                  false,
 		f(tailcfg.NetInfo{}, "DERPLatency"):                false,
 		f(tailcfg.NetInfo{}, "FirewallMode"):               false,
-		f(tailcfg.NetInfo{}, "HairPinning"):                false,
 		f(tailcfg.NetInfo{}, "HavePortMap"):                false,
 		f(tailcfg.NetInfo{}, "LinkType"):                   false,
 		f(tailcfg.NetInfo{}, "MappingVariesByDestIP"):      false,