[js] Remove old_browser define (only affects Json) (#7477)

std/js/_std/haxe/Json.hx

@@ -22,7 +22,7 @@
 package haxe;
 
 @:coreApi
-#if (!haxeJSON && !old_browser)
+#if !haxeJSON
 @:native("JSON") extern
 #end
 class Json {
@@ -39,11 +39,4 @@ class Json {
 	}
 	#end
 
-	#if (!haxeJSON && old_browser)
-	static function __init__():Void untyped {
-		if( __js__('typeof(JSON)') != 'undefined' )
-			Json = __js__('JSON');
-	}
-	#end
-
 }