http2: remove ancient build-tagged files for unsupported Go versions

x/net requires Go 1.18. No need to keep untested Go 1.11, Go 1.15, etc
support around.

Change-Id: I3588d273b543dec9ca120894ab36255f845abc20
Reviewed-on: https://go-review.googlesource.com/c/net/+/540236
Reviewed-by: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Heschi Kreinick <heschi@google.com>
Reviewed-by: Christopher Taylor <ccmtaylor@gmail.com>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>

http2/transport.go

@@ -1018,7 +1018,7 @@ func (cc *ClientConn) forceCloseConn() {
 	if !ok {
 		return
 	}
-	if nc := tlsUnderlyingConn(tc); nc != nil {
+	if nc := tc.NetConn(); nc != nil {
 		nc.Close()
 	}
 }
@@ -3201,3 +3201,34 @@ func traceFirstResponseByte(trace *httptrace.ClientTrace) {
 		trace.GotFirstResponseByte()
 	}
 }
+
+func traceHasWroteHeaderField(trace *httptrace.ClientTrace) bool {
+	return trace != nil && trace.WroteHeaderField != nil
+}
+
+func traceWroteHeaderField(trace *httptrace.ClientTrace, k, v string) {
+	if trace != nil && trace.WroteHeaderField != nil {
+		trace.WroteHeaderField(k, []string{v})
+	}
+}
+
+func traceGot1xxResponseFunc(trace *httptrace.ClientTrace) func(int, textproto.MIMEHeader) error {
+	if trace != nil {
+		return trace.Got1xxResponse
+	}
+	return nil
+}
+
+// dialTLSWithContext uses tls.Dialer, added in Go 1.15, to open a TLS
+// connection.
+func (t *Transport) dialTLSWithContext(ctx context.Context, network, addr string, cfg *tls.Config) (*tls.Conn, error) {
+	dialer := &tls.Dialer{
+		Config: cfg,
+	}
+	cn, err := dialer.DialContext(ctx, network, addr)
+	if err != nil {
+		return nil, err
+	}
+	tlsCn := cn.(*tls.Conn) // DialContext comment promises this will always succeed
+	return tlsCn, nil
+}