Remove inclusion of ext/json/php_json.h. (#3241)

That implementation of json is not being used - this extension is using
a json encoder/decoder provided by 'upb'.

php/ext/google/protobuf/message.c

@@ -30,7 +30,6 @@
 
 #include <php.h>
 #include <stdlib.h>
-#include <ext/json/php_json.h>
 
 #include "protobuf.h"