encoding/json: use zstd compressed testdata

There is a non-public zstd decoder in the stdlib (CL 473356) and
also zstd compressed testdata already present.

Delete testdata/code.json.gz and
instead use internal/jsontest/testdata/golang_source.json.zst,
which has exactly the same content:
	$ cat internal/jsontest/testdata/golang_source.json.zst | zstd -d | sha1sum
	3f70b6fd429f4aba3e8e1c3e5a294c8f2e219a6e  -
	$ cat testdata/code.json.gz | zstd -d | sha1sum
	3f70b6fd429f4aba3e8e1c3e5a294c8f2e219a6e  -

This will reduce the size of the final Go release by 118KB.

Updates #71845

Change-Id: I6da2df27bd260befc0a44c6bc0255365be0a5b0f
Reviewed-on: https://go-review.googlesource.com/c/go/+/525516
Auto-Submit: Damien Neil <dneil@google.com>
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Bypass: Damien Neil <dneil@google.com>

src/encoding/json/bench_test.go

@@ -14,9 +14,9 @@ package json
 
 import (
 	"bytes"
-	"compress/gzip"
 	"fmt"
 	"internal/testenv"
+	"internal/zstd"
 	"io"
 	"os"
 	"reflect"
@@ -46,15 +46,12 @@ var codeJSON []byte
 var codeStruct codeResponse
 
 func codeInit() {
-	f, err := os.Open("testdata/code.json.gz")
+	f, err := os.Open("internal/jsontest/testdata/golang_source.json.zst")
 	if err != nil {
 		panic(err)
 	}
 	defer f.Close()
-	gz, err := gzip.NewReader(f)
-	if err != nil {
-		panic(err)
-	}
+	gz := zstd.NewReader(f)
 	data, err := io.ReadAll(gz)
 	if err != nil {
 		panic(err)