crypto/tls: remove tls10default GODEBUG flag

Updates #45428

Change-Id: Ic2ff459e6a3f1e8ded2a770c11d34067c0b39a8a
Reviewed-on: https://go-review.googlesource.com/c/go/+/400974
Reviewed-by: Filippo Valsorda <valsorda@google.com>
Auto-Submit: Filippo Valsorda <valsorda@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Filippo Valsorda <valsorda@google.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>

common.go

@@ -18,7 +18,6 @@ import (
 	"crypto/x509"
 	"errors"
 	"fmt"
-	"internal/godebug"
 	"io"
 	"net"
 	"strings"
@@ -974,9 +973,6 @@ var supportedVersions = []uint16{
 	VersionTLS10,
 }
 
-// debugEnableTLS10 enables TLS 1.0. See issue 45428.
-var debugEnableTLS10 = godebug.Get("tls10default") == "1"
-
 // roleClient and roleServer are meant to call supportedVersions and parents
 // with more readability at the callsite.
 const roleClient = true
@@ -985,7 +981,7 @@ const roleServer = false
 func (c *Config) supportedVersions(isClient bool) []uint16 {
 	versions := make([]uint16, 0, len(supportedVersions))
 	for _, v := range supportedVersions {
-		if (c == nil || c.MinVersion == 0) && !debugEnableTLS10 &&
+		if (c == nil || c.MinVersion == 0) &&
 			isClient && v < VersionTLS12 {
 			continue
 		}

handshake_server_test.go

@@ -400,16 +400,6 @@ func TestVersion(t *testing.T) {
 	if err == nil {
 		t.Fatalf("expected failure to connect with TLS 1.0/1.1")
 	}
-
-	defer func(old bool) { debugEnableTLS10 = old }(debugEnableTLS10)
-	debugEnableTLS10 = true
-	_, _, err = testHandshake(t, clientConfig, serverConfig)
-	if err != nil {
-		t.Fatalf("handshake failed: %s", err)
-	}
-	if state.Version != VersionTLS11 {
-		t.Fatalf("incorrect version %x, should be %x", state.Version, VersionTLS11)
-	}
 }
 
 func TestCipherSuitePreference(t *testing.T) {