all: cleanup unused code, part 2 (#10670)

And enable U1000 check in staticcheck.

Updates #cleanup

Signed-off-by: Andrew Lytvynov <awly@tailscale.com>

ipn/ipnauth/ipnauth.go

@@ -9,7 +9,6 @@ import (
 	"fmt"
 	"io"
 	"net"
-	"net/netip"
 	"os"
 	"os/user"
 	"runtime"
@@ -18,7 +17,6 @@ import (
 	"inet.af/peercred"
 	"tailscale.com/envknob"
 	"tailscale.com/ipn"
-	"tailscale.com/net/netstat"
 	"tailscale.com/safesocket"
 	"tailscale.com/types/logger"
 	"tailscale.com/util/clientmetric"
@@ -207,12 +205,3 @@ func isLocalAdmin(uid string) (bool, error) {
 	}
 	return groupmember.IsMemberOfGroup(adminGroup, u.Username)
 }
-
-func peerPid(entries []netstat.Entry, la, ra netip.AddrPort) int {
-	for _, e := range entries {
-		if e.Local == ra && e.Remote == la {
-			return e.Pid
-		}
-	}
-	return 0
-}