wgnengine: remove unused field in userspaceEngine

Updates #cleanup

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

wgengine/userspace.go

@@ -123,7 +123,6 @@ type userspaceEngine struct {
 	trimmedNodes        map[key.NodePublic]bool   // set of node keys of peers currently excluded from wireguard config
 	sentActivityAt      map[netip.Addr]*mono.Time // value is accessed atomically
 	destIPActivityFuncs map[netip.Addr]func()
-	statusBufioReader   *bufio.Reader // reusable for UAPI
 	lastStatusPollTime  mono.Time // last time we polled the engine status
 
 	mu             sync.Mutex         // guards following; see lock order comment below