remove validation enforcing one Transport per net.PacketConn (#4851)

It is invalid to use a net.PacketConn in multiple Transports. However,
the validation logic is causing pain when using wrapped net.PacketConns.
It was introduce to guard against incorrect uses of the API when the
Transport was introduced, but this is probably less relevant now than it
was back then.

quic_suite_test.go

@@ -10,7 +10,6 @@ import (
 	"runtime/pprof"
 	"strconv"
 	"strings"
-	"sync"
 	"testing"
 	"time"
 
@@ -29,9 +28,6 @@ var mockCtrl *gomock.Controller
 
 var _ = BeforeEach(func() {
 	mockCtrl = gomock.NewController(GinkgoT())
-
-	// reset the sync.Once
-	connMuxerOnce = *new(sync.Once)
 })
 
 var _ = BeforeSuite(func() {